I've had my 121P for about 3-4 years now, and it is surely the best single bass item I have ever purchased bar none. It is tiny, it is LOUD and sounds great.
They didn't have the 121 extension cabs when I needed one, so I found a used Schroeder 1212L with 2x12s, and nearly as small and light as the 121P, with a similar sound. It makes a stack of 3 x 12s with 500 Watts, it is killer. Played an outdoor gig with it recently and shared it with another bass player and he was flabbergasted at the sound and power. Volume was set less than halfway, and it was TOO loud already.
As far as their popularity and marketing - My view is that
1. They broke the tiny and loud category wide open, and stimulated other makers to wake up and do the same, so now there is more competition in the same category.
2. MB is also getting a little fragmented IMO - going into pedals, modular amps and other areas that are just wasting their energy.
3. Being locked in to GC is another problem - GC is barely holding their heads above water trying to pay off the Bain Capital debt. We can only hope that the exclusivity agreement runs out soon.