Disagree, giving the opportunity to make such science powerhouses would make this top priority for most of my games. With Ranger Guild you get + happyness from animals +2 food per farm with harvesting. Together with Aristokratie +3 happines from palace + 1 food + 2 commerce from farms. Add 1 happienes from religion and a few happiness and health buildings, spam farms and there you go, you can easily have a 30- 40 city in early midgame if you dont care for unhappiness. Combine with scientific trait too build science buildings without ressources to get real OP. Not to mention sacrifice the weak in late midgame too make real big unhappy citys.
it seems to me that you are talking about a science heavy gameplay, in a grassland start, where you use your capital as a science city.: this is not the majority of the games. and it doesn't work with the majority of the civs.
hunter guild : why would it be necessary to start with that every game (opportunity cost) ? you took the science city in the capital (opportunity cost)
for other cities, you have 1 less from palace, and 3 less from aristo (and no Great library).
normally you don't have so many guilds mid game (2 ? 3 ?)
so to get more happies in science city you will need healers / or priests...
but that means that you forgoe arcane guild and craftsman and merchant guild, and slavery and Esus (again opportunity costs)
and then, you'll need to have enough food intake : will the city without grasslands/wetlands (2/3 of the starts), be able to sustain such size ?
So,mid-late game I can see that 20+ science city could be common if you took the opportunity choices that led to growing city sizes, and doing few other things.
but having size 30-40 mid game ? not so easy.
So yes, I don't disagree that it's doable... but I don't think that most games CAN even do it, and in those that would be able to do it, I don't think the opportunity cost make it such that its a generic way of playing.
And if some civs, in some situation can do this... why not.
Other game plays, with different choices can lead you to have endgame GUILD-techs and guild-revealed civ-specific units, Or Endgame summons/spells, at the same time, and be competitive.
(maybe we don't define mid-game in the same way... for me, mid game is about turn 200 in normal speed.
late game is turn 300-400 (out of a max of 600): even without a 600+science output city, at that time I often have at least 1 tier IV unit, and mages, and those units are decently promoted, and I only need to mop-up the world. ; if for you mid-game is turn 300-400... then I agree, you can get this 1000+ city mid game...
but by your mid-game I would have won my game.
and I dislike to get so powerful science cities... because science for science is not useful. What you need is science for useful techs.... and what do you do with your ultra specialized city when all really useful techs have been researched ?)
sure, you can tech to arcmages and to get all military techs...just for the +2boost but often with half of those you are already on the wining side).
anyway I'm not sure if you remember, but dedicated science cities in Vanilla FFH could also get 600-1000 science end game, if grown properly... and the tech cost were much lower.