I guess I will remain an outlier by not upgrading, even though I am a level 269 CWA right now. The only real compelling reason I see to go up is the Library research, some real good stuff there. Especially the time reductions on research.
As for why to stay, the main reasons are events, the Dominations Oil bonus, and my general desire to max out building (defensive) upgrades before moving on.
I barely finish some events now as the requirements are too high, but when they have things like the Yi helmet, finishing is worth a lot. I was able to immediately sell it and 5 star 3 artifacts with all the supplies it provided.
As for the Oil bonus, there are quite a few "rushed" space age right at 246 that are actually pretty easy to 5-star. With my museum bonuses of 60% All resources looted, and the Dominations oil bonus of +9500 or so, I regularly get 20k oil on attacks. Since I haven't quite finished Haile yet (I know I've been slacking...) and all my Generals are level 31 or under, I still need tons of oil, and oil that generally is not citizen dependent which means I can use it immediately. I've also pretty much stopped attacking Atomic bases as the +915 oil isn't worth it. Not enough return. Can someone confirm what the Dominations oil bonus is for attacking down 2 levels?
Lastly, I'll say that how I play the game has changed a lot over time. I used to login and go on huge runs, but with build times so long now, I log in a few times a day at best, attack and then I go back to my life. I prefer to make those few times as worthwhile as possible so I do less grinding, that's one of the reasons the Dominations bonus is worth so much to me. So how you play should make a big difference to how you think about upgrading.