Search:

I'm fairly certain that PSO2 was long in development and they were busy with it while PSU still got it's updates to the game. They must have, since between the
start of PSO2 and the shutdown of PSU...

There was difficulties with C-S4 with enemy levels going up to 190 I think on S4 (don't remember 100%, it's been 8 years lol), so there was plenty to do on high level as well.
Who knows what they'll...

Considering PSU ended on Lv180, it could be any level and there is no indication that they will stop at 100.
We just have to wait and see what else is coming in the future. It's still a while until...