Just to clarify what I think may be a few points of confusion here (but I could be wrong on that).
If you have Trainz Plus, regardless of whether it is a monthly or an annual subscription, you will get the TRS22 update (under the name of Trainz Plus) with Surveyor 2.0 (S20) included. You can get it now if you are on the beta stream. If beta testing is not for you (and it does take nerves of steel and an ice-cool temperament) then you will get the non-beta TRS22 with S20 when the next normal Trainz Plus update is released.
If you are not a Trainz Plus user and buy the annual subscription before Jan 15th 2022 then you will get to keep TRS22 permanently. If you already have a monthly subscription and change to an annual subscription before December 1st 2022 then you will also get to keep a permanent copy of TRS22. In both cases your Trainz Plus membership, with its regular new feature updates, will continue as long as you maintain the subscription. If you decide not to take up the annual subscription then your Trainz Plus membership, with its regular new feature updates, will continue as long as you maintain the monthly subscription. I currently have the monthly Trainz Plus subscription and will have until next December to make up my mind on getting TRS22 permanently. By then I would expect it to contain not only S20 but other new features or additional features and that will be a factor I will consider.
Everyone s different. For some users this will be an attractive proposition, for others it will not. People will have different expectations and/or life circumstances. Opinions and views are welcome but please let this not become another flame war.