I'm not sure there are any fewer MCU-style embedded jobs. Someone has to program those things.
There are more. MCU use is exploding.
And since there are fewer young people able to program in C, the demand is not going to plummet, but it's going to explode.
Now sure this will still be a "small market" compared to the rest of the software world. But ask yourself the right questions. As we said above, what do you want to do? Do you want to be able to apply for jobs that everyone else can also apply for, and thus, even if there are many jobs available, there's also a lot of competition - but maybe feel more like part of the herd, or do you prefer a niche? It's the whole question.
But the question of fearing to find no job is irrelevant. Heck, look at COBOL. Even if almost nobody is talking about it these days, and I bet you practically see no ad with a COBOL job, there's still high demand, and it pays very well. It will be the same with C.
Oh, and last word: someone really asking themselves what they should choose now to make sure it'll still be there in 15 years? It's definitely the wrong way to look at it. While I'm certain C will still be massively used in 15 years, and even in 30 years from now, you should not approach a career this way. Whatever happens in the next 15 years is yours to build. It's going to evolve. And, ironically, while C will likely still be relevant in 15 years and more, that may not be the case for other, more recent and hyped languages - so you need to reflect on that. And whatever you do now, keep preparing yourself for the next thing at all times. Just my 2 cents.