This is terrific. Thank you for starting this discussion.
I don’t think we can or should wait for individual users to make these decisions. Server admins are the ones who understand the risks and so should make this call. Guidance for server admins based on past experience (cough XMPP!) should be quite welcome.
I might refine the bit about altered API versions to really focus on the real problem: proprietary extensions. We probably want to leave the door open to try out additions to the spec that come with detailed RFCs.
But we know from XMPP that proprietary extensions are a huge problem.
When something that big barely turns a profit, I immediately suspect Hollywood accounting.
But if true, they made a game, covered their costs, left the company with an asset that can keep making sales, and probably developed their in-house talent and tooling along the way. That’s a lot of points in the “win” column.
Wise leaders understand that, in business, victory means getting to try another project with the same team, next year. Failure means disolution of the business. Earn enough years and projects with the same team in a row, and maybe you take one of the big wins one of those years.