I am way too invested so I’m going to post again - feel free to ignore!
“I’d rather he was happy and keep the friendship” - look, you’re either into this guy enough to take a chance and give it a whirl or in fact you’re not really that bothered and you can comfortably see him with someone else without being too troubled by it. Only you know which one it is. I thought at the start that it was probably the former; now I think it’s the latter.
I mean: what about the happiness you and he could share rather than this ostentatiously self-sacrificial “I’d be kicking myself but at least he’d be happy” rubbish!?!? What did you learn growing up? That you have no right to happiness?
You’re looking for reasons not to go there.
If he settles down with someone else, you may well lose the friendship anyway - a pulsating physical attraction between you and him might be a reason why he steps back out of respect for the woman he’s dating (instead of you because you didn’t want to risk your friendship).
Plenty of posters have said they or their friends have done this and it’s been great. I told you that I’d done something similar and it didn’t have staying power romantically but after a very brief bumpy patch our friendship was as strong as ever.
But you’re focussing on the few posts which talk about this going wrong.
As I mentioned upthread, maybe this kind of exciting possibility happens for you a lot so it’s easy for you to walk away on this occasion.
Maybe you’re avoidant and prefer the thought of romance to the reality. Maybe you know some significant red flags which you prefer not to share but which mean he’s not such a promising prospective partner after all.
I can only speculate as to why you’re looking for reasons not to do this. They’re obviously valid to you. To me, it looks like you’re planning to pass up the chance of something that could be fabulous in the interests of a currently okay status quo as friends, which will likely be lost if either of you settle down anyway.