[mistercow]

It depends on the target bitrate, but from what I've seen at high quality, the difference between HEIF and JPEG is comparable to the difference between H.265 and H.264.

I think your second point is more important though. There are basically two main cases for images. On the one hand, you have images which are an input to some process, like editing or publishing, where people are unwilling to sacrifice any information, lest the errors accumulate. On the other hand, you have images intended for final consumption. By modern standards, those images are already so small that saving 50% isn't usually worth either losing compatibility, or putting in extra effort to offer a fallback.

There are some in between niches of course. Apple has bet on one in iOS with phone photography. That makes sense because many people will have a large volume of images, but they're not professional photographers who need perfect fidelity. I don't know if that's actually worthwhile to most users, though, since they too have to deal with the compatibility issue.

[aikinai]

iOS still exports in jpeg unless it knows for sure the receiver (another Mac or iOS device with the right version) supports heic, or you change a setting to force it to always export heic. So users should rarely run into the compatibility issue.