I've been romancing getting a gravel bike too; everything between buying something in a showroom and throwing the parts bin at my derelict mountain bike. Of course putting the fattest tires I can on an old roadie is also a consideration. For me the issue is more the function than the form.
My definition of gravel riding is frustratingly vague. It's a combination of trails I have hiked and thought "I could ride this without a proper mountain bike", and dirt roads I've never gone down. I really don't know what it entails or if I'll actually enjoy it until I try it. Which makes investing in it a real hurdle. As was mentioned earlier, it involves a lot of the inconveniences of mountain biking - something I gave up years ago.
I know I love the efficacy of road cycling, and the efficiency of a dedicated road bike. If I had to repurpose a bike for gravel riding, it wouldn't be my fast machine. I've trashed enough bike parts and suffered enough injuries on off road adventures to make that mistake. Much like mountain bikes, I think gravel bikes will age poorly and maybe in a few years we will see the secondary market flooded with these gravel bikes that are neither fast on the road nor competent on trails.