I do actually already use Fly for pretty much everything else.

For this use case though, I forgot to mention that it needs _much_ faster autoscaling than what Fly's regular VMs offer, with unbounded concurrency, and not ideal to run concurrently in a single VM due to each request being compute heavy and needing full isolation from each other since they run arbitrary customer code.

It's true that with Lambda, some amount of cold starts are probably inevitable with extreme spikes in traffic. But I'm hoping to mitigate most of that by sending artificial concurrent traffic on a schedule to keep a decent buffer of warmed up Lambdas above the current real traffic level. Still to be seen if that plan works out in practice.