It seems like most of this works fine with browsers and MUAs separate. I can still copy text from webpages into emails and open links in tabs and add sites to my browser's bookmarks with very little friction.

Personally, and I realize I'm probably an outlier here, but I want to keep my email as far away from my web browser and the internet as possible, unless I'm being very deliberate. If I open an email, I want all remotely hosted content blocked, I don't want JS to run, I'd prefer even the most basic HTML stripped out entirely and the message presented as plain text. The more interconnected my mail client and browser are, the more I'd fear an errant click could result in a loss of security and/or privacy. Email is toxic, and I really want it quarantined.

If you're an outlier then I am also an outlier.

To my great annoyance, the text version of many emails is broken, I see unfilled template variables all over the place.