The real reason early GAs are weaker is that the
relative value of other uses of the GP go down as your empire gets more powerful.
Bulbing philo can easily save you 10 turns of direct research time and let you trade for another 20 or so (and that ignores the other very handy advantages of nabbing philo), bulbing electricity likely lops 2 or so turns off your research time and if you are VERY well positioned with respect to normal diplomacy and WFYABTA
might be leverageable into another 4 turns off via trade.
Likewise the return on settling is far better the earlier you settle. Settling a late GSp barely makes a dent in your overall espionage (assuming you've built courthouses and jails everywhere, settling one in the early BCs will give you major returns for a over 100 turns.
Even the special uses of GP become less powerful late game. One GE can finish the entire Mids in some podunk town for you, with the SoL you can only use him to play catch up. The amount of gold you get from a trade mission declines relative to your net
potential as the game progresses. Etc.
In terms of raw percent increase in economic performance, early GAs are actually stronger than late (getting 1 extra
per tile is much bigger when your average
yielding tile, like seafood, horses, and riverside farms, gives 1.5
than 5, from a respectable amount of cottages either overall or in B cap). The difference is that late game you have nothing else to do (aside from found corps). Thus your terminal 2 to 5 GP will likely end up being wasted.
All of that being said, a BC GA is not always a bad idea. The key is to make sure that it is
worth it. You are sacrificing the long term value of your future GP, but that is fine if the short term does something like say double your size. Going for a LB rush and saving multiple turns on the civic swap as well as getting the suckers out faster from increased
is worth nerfing your last few GP (who will sit around unable to start a GA for you) and foregoing the benifits or an early bulb if it will ensure you kill off the competition and dramaticly increase in land size.