This is a great thing. One more metric by which to filter out spam. If I am receiving an email from an actual person, it will be plain text, perhaps with 1-2 links or images, and will not contain any CSS.
I filter out HTML emails too, and a lot of mailing lists do as a standard practice. For some reason I find a large amount of sites which specify precise expectations for email are either German or somewhere in that region. Here's two examples:
The prevailing attitude seems to be "If you can't configure your email client to send plaintext or follow the other rules I've listed on my site, I'm not interested in communicating with you."
Well, if you ever end up working with a company for which someone created a responsive email footer, I hope your missing their email is not a big deal.
In that case, the company domain goes on the whitelist, after being noticed on a weekly glance at the spam folder. The existence of CSS headers isn't a catch-all, but is one more sign that the email is mere marketing, rather than an attempt at human communication.
I don't imagine that, no. My experience is that gmail addresses are primarily personal email addresses, not mass-marketing emails intended to be a full web page embedded into an email viewer.
Indeed. If anything is going to be an "e-mail killer", it's not going to be Slack, et al., but allowing more "freedom of design" for the senders. We'll drown in crap, and people will switch to using anything that has stronger restrictions on content (like most IMs).
There are plenty of reputable email clients that add extra styling to make email look better that are not necessarily spam.