How can you tell the difference between a "spy pixel" and an image? Whatever criteria you use can be worked around.
Email clients should not allow emails to initiate outgoing network connections... period ... and most of the good ones don't by default.
We use SendGrid to fire important, user-anticipated emails (i.e. "the Zoom class you paid for is starting at 8:30 AM, here's the private link").
Blocking all images is insane, and would set back email UX to the BBS ages. Blocking all SendGrid-served images would simply mean people host images on their own domain (or disguise the DNS), and could write their own open/click tracking systems, which would negate the point of blocking SG pixels in the first place.
I don't understand why that's bad or why e-mail needs an "UX" beyond being text. E-mail is there to convey information and text is the most efficient way to do it. If images are really important, you can include them as attachments and they'll be displayed even with remote content blocked.
... and a link that a user has to explicitly click on is not a 10th as bad as any sort of process out of their control.
That being said, I see your point. It's a game of Whack-a-Mole. What's to stop businesses from setting up a cname record on their domain to Sendgrid, for example?
Edit: Also white/invisible pixels. Maybe number of visible/non-white pixels vs. url length.