Having 100+ major factions is an edge case and not what the game is going to be balanced around, I would think. Still, the obvious solution would be to make the cost of tech be (somewhat) dependent on the number of major factions present.
*cue cries of "Now you're forcing me to trade techs*
The other point is, in the theoretical game with 100 major factions what makes you think each faction will be in contact with each other? OK, fair does on a smaller map, But, well, if one is going to cram that many factions on to a map that can't support it, weirdness is to be expected. But on the Immense and higher maps (which is where I think these mooted scenarios would tend to occur), many factions might live and die before anyone contacts them. So they probably wouldn't be able to get into a tech trade spiral.
More to the point, as you learn more techs, a "tech tax" is applied making it far harder to research the rest if you get a bunch at the same time. When I've traded a bunch of techs all at once, it then plays havoc with me trying to research my current ones. This is a feature not a bug, BTW, according to the devs.
So, yes, it might be that some races will be able to trade their way into, lets say, the Age of War with aggressive trading. Then they'll slam into a brick wall of not having enough research infrastructure to continue.
And that presumes that all of the races will like each other and not launch into wars of conquest due to not having enough room to properly grow their empires.
Mind, I LOVE tech trading, and base quite a bit of my playstyle around it. So I'm a little bit biased here.