An important point being missed here is also that there are communities (like Sc2mapster), where you can advertise your maps. As the time of writing this the custom map system has been improved vastly with the addition of many new different ways of finding custom maps, but even before that, tournament for specifics maps were organised, and maps were advertised for, independently of the blizzard pop system.
I personally took part in several custom map tournaments for the maps bounty hunters and smashcraft, tournaments who I found through the Sc2mapster community and then advertised it to my friends.
This was pretty much how it was done before Sc2 as well. Amidst a large list of custom games with unfamiliar names, I personally also turned to other websites to find cool maps. Heck, there was no way to advertise any single player maps before other than doing so through other channels then the main game itself.
The point Im making here is not that the popularity system was a good idea, as it turned out to make the most popular maps too visible, and it being too hard to find obscure ones, but this is not really a new thing. The point is that the mapmakers have always turned to other sources than an ingame tool to advertise their maps, and those channels remain open. I would propose that what would lead one to say that the map making community is "slowly dying off", is in fact that in the influx of people wanting to jump on this whole map making thing , alot of them found that it was just not their thing, and proceeded to whine about what they perceived to be the problem. The popularity system was never good, but it was never a mapmaking community killer, at worst it just limited its growth.