Think of it this way...
Minoxidil is in the class of treatments called "Potassium Channel Openers". minoxidil is somewhat of a mystery as to how exactly it stimulates growth, however it has been clinically proven that drugs which open Potassium Channels seem to stimulate hair growth. Increased bloodflow and other strong chemical signaling to the follicle to grow is one conjecture (think of it as a shot of adrenalin to the hair follicle as an analogy...a chemical cheerleader) It is not the same as a DHT inhibitor like Propecia, and does not work to stop the causes of hair loss - only to simulate growth IN SPITE of the DHT attack.
Propecia works in a completely different way than minoxidil. Put simply, it prevents the conversion of Testosterone to DHT. DHT attaches itself to receptor sites on the hair follicle and begins to treat the follicle as a foreign object in the body. During this time, the follicle, under the attack of DHT, begins to miniaturize with every growth cycle, until after several years, it simply is too small and short to be seen.
What has been shown in clinical trials is that people using minoxidil. do have some renewed growth, HOWEVER, those same people continue to progress with their hairloss.
In other words, as another gruesome analogy, suppose some hair follicles that were shot by the "bullets" of DHT and barely alive are "bleeding" on the ground. minoxidil comes along and gives them a shot of adrenalin and convinces them like a drill sergeant to "get up and make hair !!" Now, of course these poor "bleeding", DHT-bullet-ridden hair follicles are in pretty bad shape when they are convinced by minoxidil. to make a final stand and make some hair. Therefore they stand up and start making hair again at the request and encouragement of minoxidil, but they don't do it very well as they were just "on the ground" dying and are injured.
Further, minoxidil does nothing to prevent the DHT snipers from continuing to shoot the follicles with DHT "bullets". So the poor follicles make some pretty weak hair (vellus hair) and really never have a chance to regain strength and health. After a while, even the "encouragement" and "adrenalin shots" of minoxidil. are not enough to keep the follicle on its feet as the DHT "bullets" kill the follicle once and for all.
Now if you take Propecia...think of Propecia as the counter-sniper unit of the anti-hairloss army. Propecia (or dutasteride, etc) attacks the DHT snipers and greatly reduces the amount of DHT "bullets" fired at the follicle. So, the combination of the two is a very potent duo.
If you just use Propecia, standing hairs will mostly keep standing because they're not being shot at as much, but less will get up that have already fallen to the ground because they don't have the strength to save themselves, even if the shooting is stopped.
If you use just minoxidil., sure you'll get some injured and fallen follicles to weakly rejoin the fight but they'll continue to be riddled by DHT bullets and so will many times be wispy, vellus hairs.
The combination of the two is a powerful attack as explained above, as both help the fight in their own unique way.
So to answer your question, follicles that become reliant and addicted to the cheerleading and "adrenalin shots" of minoxidil are going to fall to the ground when minoxidil leaves the battlefield. Those follicles are "addicted" to what minoxidil provides and don't give a damn if Propecia comes along to suppress fire. They need their fix or they're gonna fall to the ground....
ok...now minoxidil has gone from a cheerleader...to a drill sergeant...to a drug dealer
. Sorry for mixing metaphors, but perhaps you understand why one treatment cannot "take over" where the other left off. Likewise, if you stop Propecia and the DHT snipers commence shooting full force, the minoxidil might keep them standing for a bit longer but they continue to get bullet ridden and weaker until they will eventually die as well.
These analogies are not perfect, but sometimes it's hard to imagine and understand why Propecia cannot keep minoxidil hair and visa versa. Hopefully my little grade school, crude analogy helped a bit in that regard...
