> My own VPS will eventually be broken into if I don't maintain security updates
I believe that with the right choices in life, this risk can be minimized.
E.g. tighten your sshd_config and/or lock it behind a VPN, don't expose app servers directly, don't expose insecurely written software.
> For low access patterns, it's cheaper to pay for what you use.
Low-access patterns don't increase the number of $5/mo. VPS'es I run.
> something like Heroku, Amazon Lambda, or similar, but with an open, competitive ecosystem, and without vendor lock-in.
I sense that the economic incentives lean towards vendor lock-in.
Is the amount of lock-in bad? I would have thought that migrating a function is somewhat easy.