I'm fascinated that this thread somehow became a discussion about the industry afterthought that is Leica. I also consider the whole manual adapted lens thing to be a very wonky photography hobbyist quirk, fun at home but not really very meaningful at scale. That's not to criticize doing it, as I have plenty of adapters here. It's a little different with manual adapters versus something like the fully automatic adapters like the Nikon FTZ, of course.
In any case, the professional photography industry at large is really built around sports, events, weddings, and then some secondary stuff like landscape, portrait, wildlife/birding. Really feels like studio has faded away almost entirely these days, and journalism was gutted years ago. Amateur/personal photography has basically become a lot like audio: a niche obsession for those who don't think a phone/soundbar is perfectly good. It's very difficult to understand where that's headed, as there's been a strong push towards higher margin full frame products and similarly premium lens offerings. It's hard to claim with a straight face that Sony or Nikon take APS-C seriously, and I'm not convinced Canon will for long either.
The leading professional camera company is Phase One, with over half the global market, with its origins in the Mamiya pro systems of the 1960s.
Some other older companies like Leica and Zeiss started making microscopes. Imaging and commercial and consumer optics cover a vast range of applications, whether ophthalmics (now including Nikon and Leica to Zeiss, the latter being in opthalmics for decades), sporting optics, heritage, mobile, etc. There are many other markets, such as printing, security, cinematic, logistics, all sorts of things that the likes of Canon and Panasonic are involved in. Then you have screens and other technologies that the likes of Sony and Panasonic are involved in.
Leica have about 10% global revenues compared to Nikon, but they are in a lot of partnerships, for example an alliance with Panasonic and Sigma, drones with Yuneec, mobile with Huawei, as well as their own product lines. Don't forget they invented 35mm photography. Although Leica do lots of things, they are still very heavily focused on optics. The alliance with Panasonic has been a real success in filling their gap in electronics and resulted in the SL system, a seriously good professional system.
Leica also seem to be a taking a lead in consumer optics, with their store/galleries popping up everywhere. Fujifilm had copied it with their "House of Photography" concept. I was hugely impressed when I visited. I also happen to like their cameras. Both my kids used them and my younger son has an X100F.
https://www.fujifilm-houseofphotography.com