svn on sourceforge ...
FPC svn never has been on sourceforge.
And from everything I've read about that, and still see mentioned in issue trackers, and see other posts and discussions about, that change had a very high cost.
Yes, in time. Also some things are still not up to where it was. While I was an anti GIT man (I still think it is overly complicated and unsafe), my main gripe is now the bugtracker. Mantis was way better.
Going to a contemporary devops friendly hosting platform was a major switch, and the benefits of making that switch were supposed to outweigh the cost of the switching.
I think the gitlab move was primarily driven by cutting down on own server maintenance (for SVN and bugtracker), with svn to git change, and popularity issues only secondary and third considerations.
Switching again to another platform (that likely does not offer any more capable modern devops support) is very costly (and, as I said, I'm not talking financial, though that does play a small part).
The benefits from switching would have to outweigh the cost.
I don't think switching is realistic right now, unless something really significant changes (e.g. on the gitlab hosting conditions).
Their are developers who got use to svn who do not want to use git now. That does not only affect contributors of code patches, it also affects those that find bugs and need to track them down, and for other testing and evaluation, and even usage needs.
I use GIT, but am much less capable with it, then with SVN. It is so much more complicated, and so much more can go wrong, require another level of knowledge.
The merge requests are rarely cut down to the bare minimum, and there is quite a merge request processing backlog. So IMHO having that has not been so revolutionary either. As predicted.
Some who have gotten use to gitlab would not like having to switch platforms again. And the cost of the fallout from that would have to be taken under serious consideration.
I opposed the move to gitlab then and am still of the same opinion now, but alea iacta est, we have to live with the situation no matter what the opinion. Any migration (back or forward) is not what we need right now.