Nobody know exactly how minoxidil works. Even after of almost 40 years of study, there is not a single study showing exactly how it works.
Thruth is, that the future of hair loss treatment are topicals and most closer to minoxidil, prostaglandines, etc than blocking in a systemic way the stronger androgen in males ( there is not a single chance that sooner or later you will not have side effects ) .
So, that that say " minoxidil only works for a couple of years " ask for them for an study that shows this.
Propecia is something that should never happen . It is still a miracle how the FDA approve it more than 20 years ago. Ask yourself this question " what hormone, if it is in normal ranges, can be inhibit it a 70% and don't cause side effects ? " . Even if you eliminate 70% of your cholesterol will give you side effects .
Also, keep in mind that if you stop minoxidil, you will have a massive shed that no other drug will stop it ( another thing that is not clear why minoxidil cause this ) . And by shedd I mean increasing the bald area. Once you are on minoxidil, you are "forever" with it. 20 years and counting here.