Ok, so I do not know if this is going to be read by the developers and/or have an impact on what is going to happen, but I’ll give it a shot. I would like to offer some constructive criticism and suggest improvements to what the devs are, IMO, trying to do here.
First, the relist charge. Making the order update cost more could work in theory, but the proposed fees are quite possibly too high. Also, in some extremely high volume markets, the new orders are just coming in way too quickly. But there is an unutilized feature that we could use, namely order duration.
So I say, make both the costs of setting up a buy order and that of changing the price dependent on order duration/time left.
Now we don’t want to have people just automatically relisting the item every day for a lower charge, so the correspondence should not simply be linear. Instead, let us have the setting up/changing of orders incur a fee with an additional factor f(T) and g(T) respectively, with T being the total duration of/time left on the order. (These could, but need not be the same function.)
These should satisfy that T * f(T) and T * g(T) are decreasing. So making 90 orders with duration 1 day would not generally be worth it over making 30 with duration 1 day, or a single order with duration 3 months. But setting a shorter duration would make making small changes to the orders more affordable. For example, we could have prices stay as planned for 90 months as a base rate, 0.5 times that for 1 month, 0.2 times the base rate for 3 days and 0.1 times base rate for 1 day.
The numbers could obviously be tweaked, and the functions could be discrete or continuous, that’s not the main point. A price change could either extend the duration to the original as it does now, in which case this could be discretized like above, or instead we could have price changes not reset the duration, but pay the fees according to the actual remaining time. In the latter case, actually refreshing the duration should require paying all the costs associated with setting up the order again.
This way, in high volume, quickly moving markets where you expect to sell things quickly you could set up low duration orders and have the ability to change orders with lower costs. But in low volume markets, where you don’t expect to sell the items for a while, you would just set a high duration. You could still adjust the order a couple of times without cutting too deeply into your profits, but you would be discouraged from doing so, and when you do, undercutting by a more meaningful amount than .01 ISK would quite possibly be the smart move.
You would also, as an additional benefit, add a purpose for the now virtually defunct “duration” setting.
Margin trading is very beneficial for the market as it allows goods to move more freely, facilitates setting up a supply chain for production. You can juggle more production jobs at the same time if you have a reliably working supply chain, and you can pay the buy orders from the constant revenue coming in from sell orders. If for the sake of balancing, the dev team would like to reduce its effect (by, let’s say, half) then so be it, but completely removing it seems too extreme, as many people have pointed it out already.
Margin trading scams are so rare as to be not worth considering; also scamming, ransoming and other such activities are an integral part of the “wild west” vibe Eve is going for, that which has kept it alive for so long. An inexperienced player may fall for the scam once, but that’s it, they will learn (or not, but that is not the game’s fault).
But if you really want to do something about these scams in the name of staying fair, while not protecting stupid players from themselves, then extend the tutorial with more in-depth information about the market (and quite possibly other topics, in particular fitting, but that’s not the point now). Explain how sell and buy orders work, in particular mention that setting up a buy order means if someone accepts, has the item and you have the cash, it will be fulfilled, otherwise it will fail.
So, I say, have the skill affecting relisting fees be a new skill (Advanced Broker Relations is a good enough name), instead of replacing Margin Trading, and maybe tweak the latter.
If Margin Trading was to be removed like it is set to be at the moment, or in fact even if its effect is just reduced, another way of enabling the easier supply chain management it currently affords would be to add some form of system where you can take a loan from a bank-like entity, in exchange for interest payments. This could also function as an ISK sink.
Of course, a rather complex system of additional safeguards would need to be put in place to make sure that the system could not be gamed via the transfer of cash and assets between characters. The effect could also be the exact opposite of adding an ISK sink, at least in the short term, by generating temporary money from nothing.
So we would need to have a system that ties the availability of loans to the presence of certain amounts of value in cash/assets, limiting the amounts withdrawn, restricting moving value from a character in debt etc. This would likely defeat the entire purpose of taking a loan in the first place. The difficulties would be considerable, admittedly.
So this one would likely not work, but if someone sees a way to make it work and not backfire horribly, feel free to share.
+1.
Another option, and one that does not have the same risks and difficulties, is for players/corporations themselves to start functioning as banks do. There are a fair number of Eve trillionaires who could grab this opportunity to offer industrialists/traders credit for interest. Now there might also be some, but this would have a huge incentive to be expanded and made a stated business model.
Of course, here the problem is the element of trust. So at first, the “bankers” would need to only offer to reputable traders/industrialists. Since they themselves could determine the conditions, and no ISK would be generated from nothing, the number of necessary artificial safeguards would be minimized.
Still, to facilitate gaining said trust, CCP could, for instance, add a feature to share the market transaction history of a character in a given time interval with another. This could serve as proof that the creditor has a reliable business going and is therefore likely to pay the loan back in order to be able keep it running, taking loans later as well. Also, measures should be taken that this is not gamed with character trading and such.
Once such a system was set up and working, the continued income from major industry players would enable expanding this system, with smaller, and (from the perspective of the bank) higher-risk loans available to a wider array of people.
So hey, trillionaires of Eve, here’s a potential business plan. 
I am pretty sure I missed some (or even a lot) of the implications of the changes I proposed. So feel free to tear this proposal to shreds the same way you would do to the official plans, and point out any weaknesses. In the end, the entirety of the community is likelier to find holes to fill in and ways to improve than any one person (or a comparatively small dev team for that matter).