You just answered your own question
GTA can afford to take time between releases. GTA sells well somehow even though the gap between releases can be many years.
Why would they compete against themselves?
Final Fantasy 15 is an oddity in GD terms. Because Versus 13 was announced years before it entered any kind of meaningful production cycle it can't reasonably be a game that took 7+ years. At best I'd give it fours years of generous, actual development time.
Also the development time of previous games in the series were much shorter. FF9 took less than two years from scenario writing to release and was developed in Hawaii while 8 was still wrapping up production. The only times when games took longer were due to staff or technical problems - not because they let the team take their time.
If they could they would have released these games in their intended timeframes. We really only got X-2 because XII was so far behind schedule and they had an engine ready to go.
FF13 - 5 years (development problems)
FF12 - 5 years (due to Matsuno's health reasons)
FF10-2 - 1.5 years
FF10 - 2 years
FF9 - 2 years
FF8- 2 years
FF7 - 3 years
The thing most shocking though is that while it took less than two years to develop FFX, the FFX HD remaster took longer for the team to bake. Though I guess you could fold X2 into that and it works out.