The whole "gravel bikes are a marketing exercise" isn't really fair. Any bike can be a gravel bike but if you're doing five hour rides on surfaces that are rougher than pavement but not as rough as singletrack, you want a bike fit for the purpose. Depending on where you live, you might be surrounded by lots of gravel roads and have a genuine need for such a bike. That's true for me here outside of Chicago. Also, gravel bikes are just more practical road bikes: more robust, wider tires, more relaxed geometry, lots of mounts etc. They make a lot of sense even if you're not riding gravel.
As a sidenote, I've noticed that people who say gravel bikes are just a marketing ploy simply don't have access to gravel roads themselves. They don't need one, so they assume no one needs one. This is particularly true of UK cyclists online, in my experience.