I mostly agree. I like the idea of using domains as a universal user handle, and that's part of why I have my own (going to myfancywebsite.com is still a very low bar even for the most casual of internet users), but the recurring costs (unless you could get them for like a dollar a year or something similarly cheap) and the fact that they expire and can then be squatted or exploited by other people makes them unsuitable for most casuals.
Even the casuals know about the general problems of user handles being squatted and such, and thus have little problem finding alternatives (put in an extra letter or number or two) and leaving the verification of account ownership to the platforms themselves, so the problem domains could be used to fix here just isn't that big after all.