I tend to think the modern material doesn't provide nearly as much opportunity to be colorful and provide a unique visual identity as the OG material did.
I actually achieved ✨ Inbox Zero ✨ with Inbox, and I was a teaching assistant at the university with lots and lots of emails pouring in. The concept of emails as tasks, and the features revolving around that core idea made Inbox so powerful.
Nowadays my inbox just keeps filling at a pace higher than I have energy to deal with due to the poor UI.
What really helped me deal with lots of emails was the categorization and bundling. You could sweep a lot of messages that you didn't care about. You could deal with a lot of emails in a row pertaining to a particular topic, heavily reducing context switching, etc. One of the greatest apps Google decided to retire. It really did fit my mental model of email.
Spark is pretty good, but... It just isn't Inbox. Just isn't the same.
I think thats the goal especially with android12 and above design which is super colorful and rounded to appeal to this generation and say "look we are cool wit da kids 😎"