I receive lots of HTML mail, most of them spam or newsletters I follow, and then there are those where Gmail, GitHub or whatnot adds a text/html version automatically. I just view it in the browser if it's a newsletter I expected (and there are quite a bit of them exclusively in HTML).
We can't shape the outer world, but we can transform it a bit to fit our needs. Gnus and Emacs helps quite a bit. I wish sth. similar existed for WhatsApp, but they don't have APIs AFAIK.
WRT Gnus, you should use Gmane to view mailing lists if you don't already, and with Gnus Agent you can make sure articles are available offline. Also, if you split your e-mail carefully into relevant mailboxes and use the topics mode, notmuch or alike become less necessary. I just work with Gnus and isearch, and haven't yet needed any other search tool. And my favourite line of configuration for Gnus is:
(add-hook 'gnus-summary-prepared-hook 'gnus-summary-hide-all-threads)