The average time from x.0 to x+2.0 is 8 years. If you buy a x.999 version you still can expect 4 years of free updates…
If you get it now expect 5 years. That is cheaper than supporting Ardour with 1$ per month (what I did for some time…). And if you use it rarely, you can safely just ignore updates after that time and still use it…
That is based on the past, in the future things might change…