I made a post about our experience after migrating an archived forum (ie read only - no new posts allowed) from vB 3.8 to XF 1.5 a couple of years back -
https://xenforo.com/community/threads/drag-and-drop-files-to-upload.107406/post-1263975
Summary: instant and permanent increase in visitor numbers and visits.
One of my other sites also saw a long term increase in traffic after the upgrade from vB 3.8 to XF 1.5 in 2016 - but it wasn't quite as dramatic:
View attachment 216997
This one was an older, well established site, running since 2003 and growth since migrating to XF has been sustained and is now at its highest level ever.
Despite my own experience, I don't think moving to new forum software is necessarily going to have a dramatic impact on site traffic - the results will depend on so many other factors as well.