I think it's important to use proper terminology here - SLR stands for Single Lens Reflex, a style of camera that uses a swinging (or, sometimes, translucent) mirror and a pentaprism to project its image into the viewfinder. Meanwhile, ILC stands for Interchangeable Lens Camera - a camera where you can fit different lenses onto a certain body. Most SLRs are ILCs, although not all - for example, Olympus E-10 and E-20 were DSLRs with a fixed zoom lens - but there are many ILCs that are not SLRs. Even in film days, rangefinder cameras were quite popular, and view cameras (those big bellows boxes) had their niches, and at present, most ILCs are digital mirrorless cameras, with DSLRs being at the end of their lifecycle. Both Canon and Nikon have no more DSLRs on their roadmap, Sony discontinued their A-mount SLTs years ago, and I think Pentax is still doing something with theirs, but nobody really cares. The system of mirrors and prisms was a useful crutch when you couldn't read out the sensor in real time, but now that electronics development has caught up, it is no longer needed.
Now, as for for
interchangeable lens cameras in underwater photography, I don't think they're going away anytime soon. Underwater photography is typically done with extreme lenses - either ultrawide/fisheye, or macro/supermacro. Neither of those ends is served well with typical lenses found on fixed-lens cameras, and water-contact conversion optics only go so far. That said, the market for underwater camera housings is quite small, and the field of existing manufacturers is quite crowded. At a glance, you have:
- Nauticam
- Marelux
- Aquatica
- Isotta
- Easydive
- Subal
- Seacam
- Sealux (defunct)
- Sea & Sea
- Inon
- Acquapazza (defunct, I think)
- Ikelite
- Recsea
- AOI
- Nimar (I think they went out of business recently)
- SeaFrogs
...and possibly others. I'm not listing SeaLife or the various phone housings or soft/semi-rigid bags manufacturers, as it isn't really the same market. What part of the market, in your opinion, is currently unserved by all of these companies to provide an opportunity for a new entrant?