I'm an administrator for the Roblox Wikia, which is comprised mostly of younger editors (albeit we block them when we find proof they are under 13, finding that proof is often times difficult). The removal of WikiActivity will really take a toll on our younger community who aren't familiar with RecentChanges.
The other concern here for us is moderation, specifically MediaWiki accessibility on message walls and discussions. Are there any plans whatsoever to take MediaWiki to these new platforms? The main problem here is templates, as we use templates often times on message walls to reduce the amount of time it takes to issue warnings (we have a set list of warnings and we use a template to generate the text).
We're otherwise not particularly interested in switching over to UCP until, at the very least, templates + template parameters are accessible on Message Walls. From what I've heard, we've asked politely in the past and been straight-up told "No" without a reason, which is slightly disappointing.