WTN will be implemented for
junior competition in 2021 but I don't think it could happen for adult league that soon, if ever. The premise of WTN is an open algorithm and simply switching the rating system from one number to another would introduce some issues.
If everyone knows their exact rating it would make it much easier for some to manage their rating. People do that anyways so its probably not a huge issue.
WTN also separates your singles and doubles rating so the USTA would have to create a third number which is some sort of an average. So now you would have 3 ratings.
The USTA can also control how many players they have in each league on a regional basis. They would lose that control if the rating is out of their hands.
Even officially endorsing and showing WTN ratings on tennislink could be a problem because then players would complain about someone getting bumped even though their WTN rating is lower than someone who didn't get pumped. People were doing that using tennisrecord.com ratings and the USTA simply said that those have nothing to do with them or NTRP, which was right. That wouldn't work if WTN is officially on tennislink.
A switch to WTN would probably require sliding scale league level boundaries. I think I remember schemke proposed that before. Instead of 3.0, 3.5, 4.0 and so on, you would have something like A, B, C and the boundary levels would be determined at the end of the year. That's a pretty big change and I think the USTA would want to give everyone plenty of warning if they were going to do that.