Firstly, why would I want to advertise to all & sundry that I am away from home?
I think that's a fair comment, although if you restrict your posts to friends only, it's not exactly all and sundry. I post my pictures when back.
Also, it's completely tone deaf
Is that because not everyone can afford holidays? Where do you draw the line on that for social media posts? Not everyone can afford meals out, theatre trips, new cars. If you strip everything out that might upset people who don't have the same as you, there'd be nothing very interesting left to post about
why would I be vain enough to assume that everyone/anyone is interested in me?
they are your friends, not 'everyone' (if you have your privacy / security right)
It's not vain to think some of your friends might be interested in your holiday. I'd question whether they were your friends if not - and they can scroll past if not interested. As lots of people have said on this thread, many people enjoy other people's photos.
Plus, I don't know anyone else that does this - it's just not considered "a thing" amongst my friends.
People are different. I have some friends who do, some who don't