Well seat selection is just working out you can charge for something that some care about (will pay for) and others don't (won't, largely unharmed).

But luggage for example: having it was to some extent subsidised by people who didn't/you paid for it whether you wanted it or not, in the bundled price.

It's more a problem of advertising & comparison IMO, I don't mind (quite like) these things being separated out otherwise.