I've used all of the models mentioned. Personally I've found async/await to be the most annoying. However, I think there's an important option that's overlooked

I've used all of the models mentioned. Personally I've found async/await to be the most annoying. However, I think there's an important option that's overlooked here. For many of the domains where these approaches tend to fall down hardest, notably network servers with very many connections (I was cited in the C10K paper on this subject BTW), arenas can absorb a lot of the annoying lifetime issues. They can be per-request, per-connection, per-something else, but however you do it is likely to leave you with a much smaller set of objects and lifetimes that can be managed by other means. Arenas for most things plus a few very well-isolated bits of code to handle the remainder worked for me from at least 1992 through 2017 in code bases up to more than a million lines (yes, highly concurrent the whole way) without having to deal with async/await or promises/futures. Maybe a bit of CSP/actor model here and there, but that's it.