"- UTC timestamps can unambiguously refer to times years in the future; TAI timestamps cannot, because it is unknown how many seconds will be in each year."

Isn't it the other way around, since TAI has no leap seconds?

"What would be simpler would be ending the use of leap seconds for a millennium or so."

That effectively means using TAI consistently, which is what software not aware of leap seconds would be doing anyway (despite the fact that it's actually working with UTC.)