IMO gravel bikes are like double cab bakkies in reverse. People drive them because they aspire to a lifestyle and want to go offroad over weekends and the like - camping etc with all sorts of accessories. But as cars they are not really comfortable. However they spend 90% of their life on tar and commuting.
If you put the right tyres on them you can go in to remote areas and if you bush kit them then they become very impractical for day to day use.
Gravel bikes are the same - they are really limited if the terrain gets rough when you need some suspension and an MTB does better, and limited on road. You will always be swapping tires and rims which is admin. You will always be under geared for serious road riding and have high rolling resistance. Off road your mates on an MTB will be much less limited than you when it gets slightly rough.
BUT: gravel is a new trendy thing, like fatbikes. More marketing and hype than technical merit in them but hey they are catching on. I have built and ridden a CX bike on and off road and sold that for the above reason. Great fun but other than that no real benefits, and then again two years ago built and entry level gravel bike with spares i had lying around. I still have it. The thing is it is neither fish not fowl and eventually you will find an MTB does stutter bumps better and a road bike does road racing better.
IMO gravel will go the way of fatbikes when people work out they aren't really much more than a mental image of adventuring down a dirt road, so there is never going to be one bike to rule them all - more like another reason to build a bike and have another one on the wall. Mine sits on my Wahoo.