I believe there is a common misconception that this is an actual solution to the problem of cold start latencies.

You can attempt to keep between 1 and X instances of a lambda function running, however the underlying provisioning system is mostly a black box without published details and supposedly not entirely deterministic. Keeping a single instance of the function running isn't going to give great control over the tail on latencies. This is particularly true when faced with bursty, inconsistent traffic patterns.

Hopefully the warm/cold start problem is something they will find a solution for in the end.

Well, Docker already has (experimental) support for CRIU[1]. Since they control the environment, it should be possible to prevent people from doing stuff that would prevent it from working.

[1] https://criu.org/Main_Page

And now you're back to trying to manage the infrastructure, over which you only have very indirect control.

We've come full circle, no?