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For me, the key to this is when there are inevitably sections of road to deal with when linking up trails. In my experience the riding position of my MTB encourages a different attitude towards riding, my heart rate avg will be lower every time. The same was true while I had a Surly Troll on loan, which kind of split the difference between my gravel bike and my hardtail. It was v neatly in the middle for times on my usual loop as well, which makes sense given it's rigid but flat bar I guess.
There's also the shout of using the same bike with different tyre/wheel options for different stuff, which is something I've done a lot personally.
With all the drop bar bikes with clearance for 650b 2.3” tires, meant to “tackle tough terrain”, at what point is a good cross-country hardtail going to be superior?
Even for some “gravel” races around here an XC bike with appropriate gearing will almost certainly be faster and maybe more fun.
*Disclaimer: I own and ride a drop bar bike on 650b 50mm tires and enjoy it immensely. It regularly sees single track, but I know an XC hardtail would do most of what this bike does as well or better.