okay, so in your theory, since gbg can make up the fp income, the higher A/D boost you get from your A/D GBs/AO will net you more fps, correct? my only issue with that is that you can only do a limited amount of fights per season regardless of whether you are in a good guild or a bad guild. If you are in a good guild, other players will show up when a tile opens. Thus, no matter how good your a/d boost is, it won't change your fight amounts that much. If you are in a bad guild, then you wont be able to get enough tiles to fight since you wont be in a coop. Or, let's assume you stay in plat/gold and be able to hit the tiles all the time, 10 levels on zeus/cdm/coa will give you 1-2 more attri, depending on your age. which according give you 0.35-0.7 fps. that's a horrible ROI compared to the FP buildings you think are a waste of space.
Also, FP GBs space efficiency goes up by each level, a lv 75 IT give your 1.13 fps per square, a lv70 Hagia gives you 0.97 fps per square, considering the roads. can you get better event buildings to fill those spaces? if so, can you beat them when they are lv80s? lv 120s?
lastly, if we accept your theory on FPs, you should use the same theory on your goods GBs and ditch them too since gbg income would cover them