I think this would easier to adopt if it could be plopped into an existing agnostic CI system. We built something like this in-house on top of Gitlab CI and it works really well for us. Locking isn't as much of an issue as you make it seem in the pitch, we just have our infra contaiers wait to acquire and renew a distributed lease while they're running. Some kinds of failures just release the lock and others panic and stop the world for human intervention.
Presumably your core competency isn't building CI systems or job runners so why bother? I'm sure at the core of your own infra it's job agnostic The value-add is the management plane on top of it.
The semantics of standard CI/CD providers are in practice very ill-suited to more advanced Infrastructure-as-Code use cases (triggering other stacks based on changes, multi-repo workflows, etc.), so building on top of them would add a lot of complexity. I don't want to go too much into it.
Overall, if your setup works for you and you're happy with it - keep using it!
We've seen a lot of companies (many now our customers :) ) try to build their own on top of existing systems (GitHub, Gitlab, Jenkins, etc.) and waste a ton of time and engineering resources, while ultimately not achieving anything that works well.
What Spacelift does is it gives you a bunch of much better-suited building blocks which let you build your required workflow very quickly.
And it obviously does integrate very deeply with your VCS provider - Commits, Pull Requests, Comments, etc. - everything is supported and customizable using - amongst others - Push Policies[0].
Presumably your core competency isn't building CI systems or job runners so why bother? I'm sure at the core of your own infra it's job agnostic The value-add is the management plane on top of it.