Quote:
Regular standard base mods are easy enough to replace when demolished, training mods, not nearly as much.
-1
This is why this suggestion isn't needed.
As you say, regular base modules are easy to replace at a later stage, so surely nobody is silly enough to destroy a training module already claimed(installed on your base) for another different training module when they could just destroy a regular base module.
The reason why you can't just re-build these modules(once you unlock each stage) and why they're claim once, destroy it you lose it, is because these modules are OMG WTF powerful.
With just 3 piloting simulators there are legions reaching 200k defense on their bases and with just 2 metallurgy labs some are already getting a +72% Hull Boost on top of their Auxillary Hull Boost.
So having these training modules be a 1 time installation... you destroy it you lose it thing, is the downside you pay for what are some pretty baller buffs. It teaches your legion and everyone elses, to discuss how best to handle training module installations.
So long as everyone is on the same page, none of your good training modules should ever be destroyed and none of the shitty ones should ever be installed.

My suggestion to any legions would be to have a well informed and designated training module installer. So only that person installs your training modules on the base or as the case may be, they don't install the training modules.
Tl:dr
The reason these modules cannot simply be rebuilt as and when you please, should you destroy them is because they have amazing buff potential.
4 piloting simulators with 10 trainee/grads = 80% defense buff.
4 metallurgy labs with 10 trainee/grads = 160% hull buff.
So if someone in your legion destroys them through their own volition and/or sheer stupdity, then that's the price paid.
These are just my opinions though.

P.S. You don't need to install every module researched/bought via GP or unlockable onto your bases guys/gals, there just isn't enough space to do this, even with a level 10 base, so think carefully about what it is that will actually benefit your legion the best.