That is my reading of it as well. You entered into the tenancy on the old basis, so I don't think they can fundamentally change its terms while it is still running its course.That is now my understanding as well. So as a 'large' landlord (ie. owner of 4 properties), I don't need to worry about any of the existing tenancies as the current rules will continue to apply. If one of them ever to decides to leave voluntarily (unlikely for a while), I then need to weigh up whether I begin a new tenancy (which will come with the new 'large' landlord restrictions that will mean I can't ever do a no fault eviction) or take it off the rental market altogether.
So once your tenant leaves, the decision is to sell or keep going. You could insist on short-term tenants only, of course. But a short-term tenant may turn into a long-term one. There is more churn, gaps, costs and effort as well. I suspect we'll see a lot of sales. Even the small landlord option isn't great as barring a divorce or a bankruptcy you can only get vacant possession once every 6 years. Also, even that mightn't be allowable if Sinn Fein get into government - they may impose a 'temporary' eviction ban.