Short answer = YES lots of people have trashed their RDs on MTB/Gravel."
Nuanced response: As a MTBer, I’m a ham-fisted SOB… My hardtail is on its third RD and original rubber and second wheel set, so there’s that.
My first CX bike needed a new RD from some wimpy lays-it-down issues… and I blew a Shimano shifter in a racing crash and moved to Sram to match the road bike… And the best placing I ever had in a CX race was because my short cage 10spd (running 9spd) didn’t clog up and tear off when everyone else on 10 (and a few 11s?) medium cages ripped off like MFs in mud and long grass. I literally remember passing at least 5 people wondering if I’d have caught them if their sh(t didn’t break as they stared at shredded RDs or hangers…
That bike was a true mixed surface rider, spending about 30% of its life on gravel or grass, and another 40% commuting generally, or commuting to those routes.
Your cost concerns are not invalid. It simply becomes a financial calculation that only you can make. I’m not yet in a position to feel comfortable with an RD that cost more than $275 as a replacement part. But that might have to do with saving for college for 3 kids.
But single track MTB and CX racing bracket gravel in technology and terrain, but are probably both more risky to equipment than my gravel experience.
Your cost calculations are your own, and I will 100% not judge any recreational or amateur elite rider who says “f-that I’ll ride a 5 year old group” on single track or CX racing… Or Hardcore B-roads,
But on my experience of “gravel” I wouldn’t have worried too much about the cost of replacement.
Yes and no.
IMHO your concerns about the rear derailleur specifically are overblown. Because in most crashes other things will get damaged: your shift/brake levers and handlebars are much more common to be damaged in a crash, for example. They stick out while the rear derailleur is much more protected.
Now if you get nice gear, crashes will be more expensive than if you had a cheaper bike. My previous road bike cost me $1,600 used (about $3,200) new. My new road bike was, well, much more expensive than that. (That’s not meant as a brag.) 
So if I break one of my wheels (e. g. because a branch gets stuck in the spokes), this is will set me back about 1/2 my old bike (if I can get a crash replacement discount).
My philosophy is: bikes are meant to be used, not just looked at. (Of course that doesn’t mean I don’t care for my gear.) If you want a show piece, that is fine, I won’t judge you. But if you can’t bring yourself to ride your pride and joy hard, get a cheaper bike instead. But don’t worry about your rear derailleur, just ride. Most likely it’ll be fine.
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if you’re riding in serious mud especially if it’s mud with bits a gravel in it and pretty sticky then take some time to stop and clear it out occasionally. Ideally the derailleur hanger is what will snap and not the derailleur. Or if you don’t want to stop and do that then deal with the consequences.
certainly everybody is different in how they maintain their bike. I just want to ride my bike and not spend hours cleaning and maintaining it. I rarely do a full wash. I clean the chain and derailleur and crank set. And wipe off big crud. Then maybe 1-2 a year I wash it down.
You make great points! I opted to do a GRX mechanical mullet instead. You’re right…if you’re afraid to ride the bike hard, what’s the point? I might buy a few extra hangers just in case and I can stomach losing a $110 derailleur much easier than a $370 one (and that’s the “cheap” GX AXS option 
)
I broke a Shimano shifter in the past and it was nice knowing that in a pinch I had options. Money aside, if I broke a Force AXS shifter I’d be lucky to find a replacement right now.