That all depends on whether the troops gained outside of your Alcatraz is greater than the troops that die in battle
I actually built and raised up a city without a Alcatraz. Found that up until Arctic Future (where it's currently sitting) it was incredibly difficult to run out of troops. Just kept gaining more of them due to Guild Battlegrounds. Now I'm in a Age where it's much harder to retain troops, but still managing.
Main benefit to Alcatraz is being able to choose what troops you're producing to fit your age. So if you really wanted to keep producing one specific unit, but don't have any random unit production buildings for that age..... then Alcatraz would be able to serve that purpose.
But yes, if you're not doing GvG, then you can certainly get your city to a point where the Alcatraz is a waste of space. If either of us were doing GvG, then those troops would likely be eaten up quite quickly in defending army slots. If Inno were to ever introduce something new or increase difficulty, there's always the possibility we may be wanting a Alcatraz further down the line. But under current gameplay it is more than doable to not have a Alcatraz if you're doing Guild Battlegrounds but not GvG
