It really depends on the size of the forum and the experience of the new admins.
I've worked for two Big Boards, and they would never go SaaS as they had a tech team of their own to do server maintenance to, in one instance, practically rewriting XF 1.5 to perform the best that it could, code-wise, and with server optimization to milk every last bit of performance out of 1.5 before EoL (we're talking millions of posts and millions of members).
I can't speak for them now, but signing up for a SaaS at $299 (I think that's XF's top-tier business plan) would be cheap compared to what they pay staff, but they couldn't get the immediate attention they need with SaaS unless they had a dedicated Corporate/Enterprise plan with someone available 24/7 to do something, which a tech team or dedicated sysadmin/developer can do, and is still within budget to profit from.
A Big Board will likely want full control and migrate away from anything that forces SaaS, as it also opens them up to liabilities, i.e., inadvertently sharing data that could be compromised by a 3rd party, making them liable regardless.
So, there will always be self-hosted, in my opinion, it's just whether that self-hosting will cost more than SaaS as the customer bases switch, like smaller forums going to SaaS while Big Boards need to remain on their secure stacks/own code base that they've poured hundreds of thousands of dollars into over the past decades.