Not used minoxidil , but from what I have gathered from the posts I have read and pics I have seen of people who have used it it's not that it stops working, but you just keep losing hair. Obviously what I am about to say is not science just a personal conclusion. Gonna give an example on what I think.
Let's say there are 5 situations :A,B,C,D,E. And starting from the first to the last they are in descending order of hairs in your head (You can think in terms of NW1,2,3 ect if you want)
Say 2 years ago you were on situation A, now you are on B and with your current progression of hairloss you will be on C in 2 years. If today you use minoxidil and it has a good effect on you, you will be back to situation A in 6 months. Problem is that minoxidil just gives regrowth and doesn't halt harloss. That means that after 2 years you won't be in situation C, but it will still worsen and you will be at let's say B. Give it another 2 years and you will probably be in C and after another 2 you will be in D. If you interrupt minoxidil at D you will swiftly be where you would have been without minoxidil which is situation E.
What minoxidil does is it brings some of your miniaturized, weaker and not noticable hair to normal hair, but with time passing those follicles will continue to develop even weaker hair to a point that minoxidil won't be able to grow them. That doesn't mean that it stopped working it just means that it can't do anything on those hairs anymore, but it will still continue to work on the other hairs that have miniaturized in the meantime due to your progressing Androgenetic Alopecia.
Imho to put it simply minoxidil just brings you at a previouse state and you will continue to lose hair from that point onwards. Minoxidil is like a one time time machine for hair. It brings you a couple of years back if you are lucky, but eventually time will still continue to go forward for you.