Entity forms in D8 core

I was wondering if what the pro's and con's are of making forms more entity'ish..

Why not make "all" the forms based on this form.entity type. Adding field (and thus widgets) would be a breeze (from a UI perspective). In core there are a few "fixed" forms like:
- the login form
- the contact form
- the password forget form
- the comments form
- the modules forms (like rating of add to cart)
- e.t.c.

Changing these form's isn't that easy, for example:
- if you want to add some text to the contact form you, have to make a block, and position it in the right region on the right path..
- if you want to change the order on the node add /edit form fields you'll have to change the weight by using a form_alter

As i see it, a form submit would still invoke a hook of rule to handle the validation and submitting.

pro's
- site admins have more control over the forms
- it could be staged by using the configuration management
- modules could more easily hook into it
- it fits nicely into Dries frase "Do well, Do good"

con's
- it is a major change since all modules require some sort of form

what are your thoughts about it ???