1. Does such a beast really exist?
2. My goal is to ride old, poorly maintained asphalt, flowing singletrack, and some hilly routes around where we live, as well as some general-purpose urban riding.
3. I'm not interested in drop bars or carbon. Flat bars are a must; aluminum is fine, but (affordable) steel would be really nice!
4. In terms of geometry, durability, and equipment (and upgradability,) how far from (or how close to) a true XC bike does a quality, well equipped hybrid fall? (Think along the lines of the alloy Trek FX gen. 4 or Trek DS gen. 5.) Will either of those types of bike do for my intended purpose instead?
5. Is it really a flat-bar gravel bike that I want but I just don't know it yet?

I'll ask more questions as they come to mind or as inspired by the replies.

I have three full rigids ,two of them have a flat bar, one from the early 90's that I restomodded with XT T80T Transmission an v brakes non slopping geometry but very sure and stable

Another from the mid 90's with a slightly sloping geometry, Scapin made of Dedacciai 18MCDV6HT, it was a MTB that I have had built from scratch. It is equipped with XT T80T Transmission an v brakes

Both are very good bike , the Scapin being lighter. They do very well on single tracks, rocky terrains, poorly maintained asphalt and hilly routes.

You can always just put a rigid fork on a hardtail MTB. Seems to be a popular swap when people are putting together flat bar gravel bikes.

Soma sells some rigid forks for 26” as well as 29”.

I would personally just buy a gravel bike.

new or used ?

used - one of a zillion vintage MTB’s … but they barely compare to a current contemporary XC bike

new - one new bike that comes to mind is a Jamis Sequel … but I don’t categorize this bike as a rigid XC bike - and I don’t know if one exists ?

https://www.poseidonbike.com/product...43754995286244
Odd ball bike but works for what you want and cheap only problem is they seem to be out of stock atm unless you are huge.

Drop bars are out. If it's going to come down to putting flat bars on a gravel bike vs. putting a (suspension corrected?) rigid fork on an XC bike, I'd rather go down the latter route. That way at least I'll know the geometry still works.

I've always been one for the oddballs, the outliers, the weird and funky ones! I came across the Redwood some time ago and it did pique my internet for being somewhat outside the norm. I'm curious as to their choice of short pull brake levers and calipers on a flat-bar setup, though.

Disk brakes and indexed shifting rule out vintage MTBs...

Indexed shifting came out in the 80s just FYI.

As above, you can find tons of steel MTB's from yesteryear that will suffice with flat or rise bars and rigid forks. FME, the ride will be slower or much bumpier the more rugged the terrain than suspended units. Two of my examples both which were motorized at the time but functioned perfectly.

If you're not after blistering times then rigid fork MTB is one possibility. There are not many of those available off-the-rack. Adding a rigid fork of the appropriate ATC to an XC/trail bike is one option.

There are some decent mid-90s MTBs that make for good flatbar gravel bikes. Right around 1997-1998 was when lightweight high-quality rigid MTBs started disappearing. Trek/Gary Fisher/Klein, Mongoose, KHS had some pretty nice ones in the 25-pound range. No disc brakes though. There were plenty of light hardtails from the era also, which you could put a rigid fork on.

For a modern flatbar gravel bike. Not many choices off-the-rack, but the first one that comes to mind is pretty awesome (price no object)

https://www.scott-sports.com/global/...gravel-rc-bike

Jones Bikes has entered the chat

LWB (Long Wheel Base)



SWB (Short Wheel Base)


SWB Spaceframe




https://jonesbikes.com/

Wish I had a better pic of my old Mongoose. 25 lbs when I got it, 24lbs with lighter cranks, bars, tires, that thing flew!!!

Saw those a while ago and liked them, I even watched all the videos by Ride Year Round and others. Not sure how I feel about those "alt" handlebars, though.

I was not a fan of the Jones bars (too swept back for my tastes) but love alt-bars like the Velo Orange Crazy Bars (V2) and the Koga Denham bars and the Surly Moloko Bars, really sensible bars for riding as you have multiple hand positions. a nice 34 degree sweepback seems to be ideal for my hands and wrists.

Jones bars have too much sweep for me also. I prefer pretty simple bars for most applications however,

Soma Dream bars are my favorites. There are also versions with more or less rise if needed.

Here's the originals:
https://www.somafab.com/archives/pro...iser-handlebar

I've read some critiques of the Jones bars too, bit I also read that the bars are designed to work specifically with the Jones bikes. I don't know the numbers but it's probably related to stack and reach.

My back and arms hurt looking at some of these photos.

Where would run your bars?

There's a rigid build of the Surly Karate Monkey and Krampus available, and their dedicated single speed BMX/MTB Lowside still has an RD hanger. They still make the Ogre too, and the Bridge Club which is kind of a downgraded version for less dough. Similar bikes from other companies come and go, like the Stache. I don't think I'd call any of them "XC" though.

Actual rigid steel XC racing bikes without a suspension fork, you'd need a time machine to the mid 90's.

Some of the other styles of bikes suggested here like gravel and hybrid are not heavy-duty enough for your description, and usually don't take big enough tires. Though I guess it depends on your riding style. If you pop off and over things, no.

Soma as well has steel frames and sells rigid forks. Reasonable prices as well,

https://www.somafab.com/archives/pro...e-frame-a-type

Yeah, but they haven’t sold completes in some time. Too bad, they were nice spec.

Take a look at Hudski bikes, they seem to be aligned well for the type of riding you're describing:

https://hudskibikes.com/collections/bikes-framesets