From a system engineering standpoint we are looking at the tech tree representing a quasi fixed amount of research to complete. I shall call this quantity "total tech yield." The question posed is dealing with players finishing the game too quickly by leveraging several methods.
Indeed, around 40% of the "total tech yield" required is bypassed when you complete eureka "quests".
Since a lot of these quests are tied to production, it is much easier to not only complete them, but complete them in a shorter time window, when you own a larger number of cities as you have:
higher total production in your empire (of which you can diversify against multiple eureka quest goals)
higher total of district slots available (allocating 3 slots to IZs out of say 20 is a more impactful decision than 3 out of say 80)
& more features available to harvest/chop (each single chop decision becomes of less significance).
So not only does the
tech rate area (ie campus spam) benefit a wide empire more, but the eureka/inspiration system does too.
I would almost rather they boost specialists or at the very least come with an increase in maintenance, which ties into...
Increasing the economic cost of operating tech rate increasing districts/buildings/specialists will require more other things to support it than it does now. While this reduces tech rate area by limiting how many campuses you can spam, it also ties tech rate area to a sufficient economic base, which requires time to develop. If you needed 3 cities worth of CHs and trade to support 2 cities worth of Campus+TS, then you have radically changed the paradigm. (Example ratio, of course.)
This is a straightforward and simple way to reduce and delay total science output, but recently I am trying to move away from such a solution as I think it is "bad game design". It simply makes build order more rigid (ie you simply need to build CH/Harbors before you build your Campi/Theater Squares). But what if I want to play a Jesuit Education game? Faith generation also costs some gold to produce, so now I need a Holy Site, CH and Harbor before I build a Campus or Theater Square? Running international trade routes instead of internal ones also becomes more of a necessity to generate the necessary gold output. Limiting the number of viable strategies or approaches is not a good idea in my opinion...
And most importantly in this example, the science/gold ratio needed stays the same independently of the number of cities you have in your empire. Whether you have 6 or 20 cities, you need 3 CH/Harbors for every 2 Campus/TS.
Currently, a player focusing on
tech rate area is greatly rewarded (and adding another campus is never a bad decision, except maybe very late game).
Currently, there are not many ways a player can work toward increasing the
tech rate density of individual cities.
Now, I think an interesting idea would be the following: decreasing marginal benefits (decreasing marginal science output) for every increase in
tech rate area (total number of campi).
Thus, there would be a point where a adding another campus (versus another district) would become a mistake.
