if you have, for example, 4 TFarms at level 12, that would be 48 FPs in 120 squares. if you want to have most part of your city with attack buildings, you are losing 120 squares. level up your hagia up to 80 (23321 FPs) gives you 48 FPs in 42 squares, then delete the 4 TFarms and you will have 72 free squares to put more attack buildings. if those buildings are 3x3, then you would be able to put 8 more in your city + the roads you can delete because hagia is only 1 building but the TFarms are 4.
people who say "I don't build that due the RoI really don't know how to play this game XD
Or they just have a different objective than you in the game. Where FP is a means to whatever their preferred end is up to a point. And once they're past it FP kinda loses meaning. Not everyone's in it to have the highest GBs. In that concept RoI has meaning. Because 3-5 years down the road when you might finally be ahead for having built something, you may no longer care. So a 3 year ROI: not worth it to that player.
There was a time when RoI would've even supported building Hagia btw - probably did around the time you put yours up. Its footprint didn't really used to be much of a consideration - it would've been taking the space of some extra production buildings or some crap. But that doesn't mean the math is the same for a newer player who has yet to build one.
Let's say preconditions for considering building Hagia are to have already done Arc to 80, have access to a 1.9, and have done Cape Canaveral to 60-70 as well because it does the same thing as Hagia, just much more efficiently (smaller and makes more FP). If by that point your city is not yet already close to full of good event buildings, Hagia may still have a place for you. But if you have good event buildings (and there is much better than L2 terrace farms you chose as a point of comparison), then the marginal extra FP provided by Hagia with its gignormous footprint are probably a low priority.
The one case where I'd still understand someone building it now is if they were trying to rank up in global rankings via GB levels and I'm not sure that even really works anymore given how many battle points people are pulling in. In that situation Hagia makes sense on the grounds that it's an extra sink for FP that has less cost per ranking point (especially over time) than the stuff you've already levelled way too high.
On the other hand the flip consideration is : having already built it and levelled it, when will you delete it? To which all I can say is "not yet", but it's plausible someday.