I'll bite on this...
MF is only a niche market now because of the cost and design. Most reputable systems cost well over 10K, and, they are much bigger than an slr and the range of use is fairly limited. If your shooting MF now your either in the upper 5% of photographers, and your working mostly in a studio, or on location with a team of assistants and a lot of lights. Or your shooting epic landscapes (but come on now, how many landscape photogs are making 300K+ a year, not many. Most using MF are working with large scale ad agencies.
Limited function - most MF rigs max their ISO at like 1600, and I have never read anything from a MF shooter that promotes using an ISO over 400. I don't think there is an MF rig that has a burst mode. You aren't busting out the MF body to shoot a wedding reception. Your not busting out the MF to go shoot live music. Your not busting out the MF rig to shoot sports. Wedding, Event, portrait, and art shooters fill out that remaining 95% of pro photogs.
Last I checked the average yearly salary for a working photographer in the US is $29,000 a year (of course this is an average). MF is way out of the range for any of us at or near that average.
MF will continue to be a niche product as long as both the cost and design of MF is what it is. Now with that said, I'd think that canon/nikon/sony would differentiate from the current MF offerings - which may mean compromises - less IQ than current MF but better AF, burst modes and increased ISO also offered a a substantially lower cost. Again, there are only so many working photogs that can afford even making the leap into something like a 1dx. So even if it's priced at 1dx levels there will still be many who say no because the investment doesn't make sense financially
From a wedding photographers perspective - a MF rig would probably only be seeing use for the formal portraits of the bride and groom (or,yes, the first dance but you're only if your in the upper crust where you have more advanced lighting and multiple assistants with monopods so you can keep that ISO low). And you better be selling Large prints - or catering to a clientele that either would notice the difference or shooting for the extremely rich. Mind you, even if sony/nikon/canon did enter the MF market and offered cheaper ---let's say 6K - that's just the body! Lenses will be costly. this narrows the potential market down a lot when you consider that many of us would have to devote a a good portion of your yearly salary to the investment!
As I see it, those are the bottlenecks to MF becoming more than a niche. It's just too costly to be a reality for the bulk of the market.
In 10 years though.... Yeah, I can see the future phasing out APS-C in all DSL bodies, all FF. By that point the cost to make FF sensors will have been streamlined to a point to make that happen - and if that research is added to MF sensors, you could see a cost reduction and I could see some amazing things happening...
But ---- there is one other factor that adds or subtracts time from any of the above - the global economy needs to turn around. More costly systems can't fly when disposable income is low. Less on hand money for the average person means less purchases of things like...photography. People aren't buying all over the place so it's a ripple effect. If the economy turns around then I think we'll see another boom in tech offerings. Without that companies are bound to be more pragmatic.