Two years difference can be caught up fairly easy. They just release the story maps twice a month instead of once a month and they'd catch up in that aspect in 2 years.
For events they could just not have huge breaks for events and run it at the same time as CB and they'd catch up with the events too; same with luna tower run an event while that's going on. It'd be the same as story, could do it in two years easy enough.
Gacha they can speed up the the banner cycle and skip the 2* banners, rearrange seasonal and prifest stuff to fit. Might take about 3 years to catch up doing that but still doable
Fact is they didn't want to speed it up and it's fucking cruncyroll they'll probaby scrap the game in a year.