I certainly see the appeal of a gravel bike, we have plenty of hard pack gravel roads that would warrant their use here, but given I race MTB XC on some pretty demanding courses, a gravel bike holds very limited appeal to me - my 29er is faster over 60km and 1200m of single track rocky terrain and is far from boring given how technical some parts are.
I think it depends upon where you live and the local trails available to you. To me a gravel bike is simply a road bike suited to dirt roads and blurs the lines with a CX bike. Nowhere near the same as MTB'ing in my local context where we have very technical challenges. But then I agree that MTB's on easy trails, given how capable they are, could be considered boring in such environments.