Reviewing those PRs require engineer time from a HashiCorp employee - how many resources should be diverted to it?

It need not be HashiCorp employees. I merged as many (if not more) PRs into Terraform after leaving as I did while working there - the notion of community maintainers was nixed in 2018, though that was never communicated.

They actively decided against free labor? Wow.

Control over the product direction is more important than free labour

> Reviewing those PRs require engineer time from a HashiCorp employee

No it doesn't. There's nothing stopping them delegating that task to community members apart from the desire to retain corporate control.

Corporate control has most likely been their game plan from day zero.