I always assumed this was intentionally done this way so that people click the “display external content” warning and inadvertently activate the tracking pixels.
The responsible thing for services like gmail and o365 to do would be to eat the resource costs and just open ALL external content within a privacy sandbox, thereby polluting the data. Then you can re-enable displaying of images for everyone and the experience for designers and end users gets better.
What it can’t do sadly is strip out tracking functions (unique IDs and such). Dunno how that could even be solved tbh
What the parent is suggesting is for Gmail to download the images as soon as the server receives the email, thereby rendering any tracking data meaningless.