Date: 2019-05-02 16:38
>> machine them down so they fit the socket like a tuning slide on a brass instrument. <<
At least here, if I machine them to that fit, they would get very resistant, if not impossible to assemble and disassemble by the player, as soon as they play or weather changes. Even a looser fit than that, by not enough, can make the tenon too tight part of the year, or with enough playing.
Most of the time I use the method Chris described. I've never had a problem with it. Sometimes it can even save a significant amount of time since the cork doesn't have to be replaced (if it's good).
I've made sleeves but it takes longer and after more than a few, I think it is rarely necessary. If it's for the inner shoulder the cork would have to replaced too.
I've also done the metal tenon rings, using both silver and brass (I prefer silver). It just takes longer and I don't think it's better really, so more often than not it's not worth the effort.
>> Replacing the entire tenon looks like very major and specialized surgery <<
...and completely unnecessary for a loose tenon. It's like replacing a panel on your car because of a scratchThis is only necessary when a tenon or socket break off, and even then in some cases it's not.
A heat shrink tube is likely to peel the moment you assemble it and almost certainly is too thin or too thick anyway.