In the U.K. there were, and still are, new bikes in dealers stocks. So the bottleneck doesn’t appear to be inability to manufacture enough volume, it is an inability to sell them.
You could easily say this about MV Agusta, Moto Guzzi and Aprilia (and a few others) in the US. There are plenty of new 2018 & 2019 bikes of those brands in stock at dealers - even quite a few new 2016-2017 Moto Guzzis.
That’s where the quality comes in, stories of poor quality put buyers of in droves. It caused dealers to drop the brand, other dealers not to take up the brand, etc, etc. I’m fairly confident that if these bikes we’re fault free, dealership networks in the US would have been forthcoming.
Dealers do tend to get frustrated in dealing with unreliable manufacturers, particularly if they get strung along on promises. MV Agusta lost a lot of dealers over the years due to lack of reliability in getting parts. Dealers were tired of selling bikes they couldn't repair under warranty and having to deal with disgruntled customers. I believe Norton had 15-20 US dealers at one point before all but 4 gave up on them too.
MV Agusta, Moto Guzzi and Aprilia (and others) have survived not because of great build quality or reputation, but because there are just enough people out there who want something a little more unique than US and Asian brands, and because the management teams of these companies haven't made really poor (for the most part) decisions in sustaining the organizations. And have managed to get capital inflow at the right time to survive (MV Agusta). It seems to me that the majority Norton and MV owners will put up with a certain level of issues with their bikes to enjoy the products. Also, I tend to see that people who own multiple bikes (and people whose bikes are not primary transportation) put up with more issues, as they have another bike to fall back on.
Norton and MV Agusta have some similarities. New Nortons are viewed by many people I know in US as just being premium Triumphs. MV is viewed as just being a better looking Ducati. Neither Norton or MV Agusta have any real technical, build quality, or reliability prowess over Triumph or Ducati, but there are enough people who just want to try a 'perceived' premium or exclusive (read higher priced) product, and/or want something that is just a bit different than what others ride.
I've seen MV nearly fail for trying too hard to compete with Ducati by attempting to scale up volume and offer too many models. When Norton tried to scale up with the 650's at the same time as the V4, it seemed to me they were heading down a path of spreading themselves a bit thin. Bit off a bit more than it could chew perhaps.