I would also add SWI-Prolog has some commercial users whose products heavily depend on SWI-Prlog, see this article:
It is not just this or that side project, so they have considered the risks and are using SWI-Prolog. They have funded the development of some features in the past.
You also should consider your ‘threat model’, what are the actual risks? I would say that with SWIProlog the biggest risk is simply ‘If we hit a bug will it be solved?’ If you don’t use the new features (like tabling) the probability of you hitting a bug is rather low, but nonetheless if you look at the commit log there are other people that have solved bugs even though Jan was the one that committed into master.
What I wrote above is supposing that your team/company is already sold on using prolog, if that is not the case we’re in a totally different game.
EDIT: if the choice is restricted to open source prologs, I honestly don’t think any other alternatives come close to the usability, maturity, features, and support that SWI-Prolog has.