Re:
While I'm not disputing the fact 'the Industry' likes to literally reinvent the wheel for profit every few years, there are a number of points in your post that I don't think truly represent what a gravel bike is:
They are quite frequently 650B, not 700c (though there are plenty of 700c ones. obviously), this comes mostly from people like Jan Heine riding vintage French constructeur bikes with the 650B wheel size than anything to do with Specialized/Trek et al trying to flog the 'new' MTB wheel size.
The name is a little disingenuous, but over in the states where the trend took off there 1,000s and 1,000s of miles of gravel roads, some made famous on long distance races like the Tour Divide or Oregon Outback. We don't have a lot of gravel in the UK, but you can hunt it out, either way, the term 'all road' is probably more relevant.
Most modern (last 15ish years) road bikes with 700c wheels won't take a wide tyre, you can't just slap a 38c tyre on a bike with clearance for a 28. Touring and cross bikes will, and they are what they Rough Stuff Fellowship etc have been gravel riding on since the 50s, but with the advent of cheaper, lighter discs and people wanting to ride bikes fast over rough roads an old steel road bike probably wouldn't cut it for them.
A gravel bike and an MTB have some similarities, but quite a few key differences: drop bar, geometry, weight. Not that you can't use an MTB to ride gravel, but would it feel as quick on the road? An all road bike works well on lots of different surfaces, not just off road, and it certainly wouldn't be my choice for trying to get down a black run at a trail centre quickly and in one piece.
Anyway, back to the OP's question: Yes, I have, but out of a early 00s 26" wheeled steel touring bike, not a 90s NORBA geometry MTB. And you can stick to MTB, but if you want to ride off the road, but not 'OFF ROAD!!!' and cover some miles, quickly, then a gravel (all road) bike will work, yes.
Please note, I am an addicted serial bike builder, I need very little in the way of justification to start building another, so it made sense for me to build an all road bike, even though I have a cross bike, touring bikes and MTBs already. I have no regrets, it's beautiful and rides nicely, your opinions may differ.