I think using what you already know remains a choice that is very hard to criticise. But we didn't have to learn that, did we?

Beyond that I think there is more to unlearn than to learn from the history of the Y2K batch of startups. The economics of essentially everything related to writing, running and distributing software have changed completely.

In part the reason that PHP8 (and it was 7 that had the quantum leap in perf) are now so fast is precisely because of Hack - it was easy to accept the status quo on performance until Hack showed there really was a lot of performance left on the table.

For me the biggest win was the changes they made to how arrays are stored in memory, I saw some systems drop by half in memory usage and had to change basically nothing - those kinds of wins are rare.

Yeah, I know the performance optimization were in part because of hhvm.