If you intend to put the structure up where others have a public refinery they have an incentive to knock yours down to make their own more attractive to customers.
If you put it up somewhere out the way it will go longer without being noticed.
Local competition is one incentive, but in answer to the original question, “because it exists” is certainly another. Advertising that it has a T2 large rig makes it more attractive to those who like to kick down sandcastles.
Hiding it might work for a while, but you’d be better off finding a null sec alliance that would (a) appreciate your offer and utilise the benefit and (b) help protect it from prying eyes / attackers.
(As a fellow solo player I know how unattractive that may sound.)
There are a certain number of groups blowing up Upwell structures.
I would suggest that at some point it will indeed be blown up by someone because it exists. However how long that takes is down to the placement of the structure and of course the appetite of structure grinding war deckers for another grind.
You might be better off just waiting until your athanor gets blown up, though if I was you I would likely just build a larger number of Athanors and equip them to a good degree and replace every so often.
It really depends on where and the sort of local competition.
If you put it 1 jump out of Amarr it’s probably not long for this world.
If you put it anywhere else then I doubt it will pay out enough to be worth it.
If you’ve got your heart set on doing it I would look at mission hubs in or near 0.5 space, throw it up on a decent moon, and do public mining pulls to attract the right sort of attention to it.