I apologize for some nonsense terms I may use, I'm a Designer, not an advanced developer, but I really want to contribute in some way.

I agree, Page Templates as used before in the page editor seem redundant with Patterns coming in.
But I think keeping the Template Hierarchy is fundamental for building dynamic Websites/Apps/PWAs.

I also think WP can have two kind of users:
A) the ones who want to build single pages with blocks.
B) the ones who want to fill fields to feed a Database - that would feed the template-pages & blocks.

A) could be divided in Normal and Advanced users. See structure below..

Thinking on a data structure, I believe some kind of CPT and Custom Fields are still essential to build Complex Queries. And as data gains more value it's important to control it. Of course, with blocks the way it's organized it's different but I think the a good approach could be:

AdminMenu
|__ Dashboard
|__ NEW
| |__ Page (page-template.php) - Pattern X, Y, etc
| |__ Product (product-template)
| |__ Post (etc)
| |__ Etc..
| EDIT Templates/Patterns (Advanced User level)
| | Page
| | |__ Pattern X (Header A + Sponsor A + Etc)
| | |__ Pattern Y
| | |__ Etc.
| |__ Product
| | |__ Pattern X
| | |__ Pattern Y
| |__ Post
| |__ Etc..
|__ EDIT BLOCKS
| |__ Header A (header-template-a.php)
| |__ Header B (header-template-b.php)
| |__ Sponsor A (sponsor-template-a.php)
|__ Media
|__ Etc..

Where "Templates" option would only show in Dev or Advanced user level, controlled on WP installation and after in some kind of on/off switch.

Blocks themselves should be simple templates, dynamic = PHP (dynamic HTML).
HTML isn't more user friendly than PHP - a user will never touch code anyway. Why not having PHP doing the heavy stuff - fetching, parsing, embedding, using custom-fields, etc..?

PHP is so strong now. It had its down momentum with the appearance of new systems and languages but it's being updated more than ever, it's quicker, safer and it is a super SS language. With the evolution of CDN's, Cache solutions (functions?), etc.
Isn't it a strong option to keep it alive?!

I would prefer go 1 step back now then 2 or 3 later.

I hope it wasn't to confusing or absurd.