Building high-quality test infrastructure often requires more skill than building the software it supports. Some of the most impactful work I've done in staff/principal roles has been test infra.

Yep, very very true. Unfortunately some of the places I've been at more recently have believed that mock servers are the best way to do end to end testing. It was a battle I couldn't win, but no one was willing to see that testing critical UI flows against something that spits back canned JSON could hardly be considered thorough testing.