Plex enters EVE when some player decides he wants something (ISK, NES store, SKINR etc.) that can be exchanged for Plex. So at base, it’s limited by “how many players are willing to send real cash to CCP to get something requiring Plex”. This number has been declining for years, for various reasons.
Plex purchased does not always go to the market - sometimes players pick it up on sale for later use. Some people buy it on the market when they have excess ISK and see a dip in Plex prices. Some people trade it regularly as a commodity, some people hoard it, some people set it aside as a store of value/ISKflation hedge. And it’s fairly clear that some large traders/speculators occasionally manipulate the market when supplies are tight and push the price up higher before unloading some for a major profit.
That’s all standard EVE market economics and is player driven, not CCP manipulation.
In the long run (I’ve played EVE almost 2 decades now), “less Plex entering the system” is my opinion due to a number of factors just from my personal observation (too many factors to go into here). However for a “real numbers right now” reference, I check here:
https://www.adam4eve.eu/commodity.php?typeID=44992
Look at the second chart, Order Item Quantity. You can see that ‘Buy Volume’ (how many offers to Buy Plex on the market) has been trending up for the past year. ‘Sell Volume’ (how many Plex are offered for sale) has been mostly trending down. So, more offers to buy, fewer posted offers to sell, generally leads to an increase in prices.
When we’ve had similar buy/sell bottlenecks in the past (eg. 2019), Plex prices rose dramatically. My personal belief there is that sharp traders notice a period where less Plex is on the in-game market, they start buying up Plex to hoard it, which leads to even less supply, driving prices up, then they unload it after the price increase for a fairly massive profit.
That’s short term trading though. Long term, there are fewer players adding new Plex into the system overall, and more multiboxers and “FreeMega” accounts looking to Plex their accounts, and more overall ISK in the system every month. So prices will always trend upward.