"C1" server doesnt really exist per say, C is for the client
. Altho either way, c1 is never coming back, not because it was 2d or any similar reasons, but merely because c1 code was a total and unrecoverable mess. I was not a big fan of the 3d nature of c2 either at first, but orthographic camera pretty much fixed my only annoyance with it. By using 3d rendering it made it a lot faster to handle camera related operations (zoom in / out) as well as more performance in effects like dynamic lighting, particles, and objects in general.
While in the nostalgia goggle view, c1 was high performance and everything, it most certainly was not. Still to this day, most of the slowdown on c2 come from code that was copy pasted from c1, the rendering itself is usually negligible when compared to the logic that happen to make the scene, that logic only improved with c2 it did not get worse. We constantly push the limits of the client and improve as we hit those limits, thanks to c2 cleaner code, we can actually push client updates. Theres a reason why on c1 we tried to do every single thing on the server even when it made no sense to do so, the same reason as for why in the last 3 years of c1, less than 5 client patches were done and usually those were pretty small. Any change we did to it usually came with major problem from old spaghetti code and corruption creating more crashes than good.
Back in c1 days, players were a lot more used/forgiving toward lag, hell for well over 2/3 of starsonata existence, the server ran at over 300ms per frame with spikes in the 1000+ms multiple times per minutes and people just barely whined about the lag (that was a 300-500ms of ping right off the bat + the spikes.
Now the server run at around 30ms with lag spikes every now and then in the 300ms and when those lag happen i get players complaining about the lag left and right. The rate at which c2 get the updates on everything is as close to realtime as you can nowadays in an online game (we use same kind of network rate as most fps), c1 would lag to death just trying to play catch up with all the network data on the old code lol.