I'm not a developer but I imagine that having it deplete or not deplete is fairly simple to code, compared to making the rest of the game.
In other words it could be tested fairly easily I *believe*
Could just be wishful thinking .
Ofcourse the trick is in assessing whether it makes the game more or less fun.
I favour cosmite depletion and/or diminishing returns but my thinking is heavily heavily coloured by considering how to make T4s good but not gamebraking, and how to bring a game session to a natural close without the dreaded end game tedium.
Depleting a resource as valuable as cosmite acts as a timer of sorts.
It also gives players locations of real value to fight over, strategic objectives that aren't cities, although ofcourse I hope cities will be useful.
So that's my support for depleting resources.
There are other mechanics that can help here, all worth exploring, like upgraded sectors, multiple t4 types, differing victory conditions etc etc.
As an aside, I hope they'll keep in empire achievements, like centurion (first to 100 units) and paragon (1st to have a t4.)
Another aside- I'm hoping that, related to depleting resources, that there are variable income mines etc (or whatever produces energy etc) and that you can upgrade these resources, again in service of giving people juicy targets other than cities