It's my fault then for getting that impression, I apologize.
I play on high difficulties (single HW only) and maintaining Fortresses hasn't been a problem for me. You've made threads before with concerns about something being unfair on difficulty 10, and since you play on 10 a lot, and didn't specify which difficulty you were playing on this time, I assumed it could be 10.
The only thing I'll say on this topic is that if people are unlocking MK2 and 3 Fortresses, and realistically attempting to place all 3 tiers, they have a fundamental misunderstanding of how the Fortresses work. I wouldn't unlock MK2 and 3 Fortresses just to have more to place, I would unlock them because, for their costs, they are better. I'm not sure if that's the concern, but I wanted to throw that out there.