It's a lot easier to test and prepare for something that occurs at least semi-regularly than a monumental event far into the future that is "so far into the future we don't have to worry about it"....
Imagine what would happen when the International Date Line ends up in the middle of the US.
The Earth's rotation is slowing down (by about 2 ms per day per century), so the difference between TAI and UT1 is increasing quadratically, not linearly.