Gravel / Gravé / Gnarmac / Groad / ATB

I've done my best.

This is the smart answer. Bar-enders since you should be riding in the drops a lot of the time anyway.

I've been using the 10s MTB version since 2015, it's a bit shonky, the index indent and the actual shift points don't line up to make a great shift if that makes sense but it's not so bad that I've taken it off. It's not work or got any worse in all that time and it's had a lot of use. I think the Sturmey bar-end @Alb had on his bike was the better option though - not sure if that was 9 or 10s and they prob don't make a 12.

In the hope of getting someone else to chime in, I think your estimate is correct for the tyre, but ignores the difference in wheel (bigger rim, bigger spokes) and rotor (as ed said, you need bigger rotors for 29ers)

If you take fortus 23s, there's about 100g for two wheels so...I guess... including rotors...maybe 200...250 grammes for a pair of 27.5 vs 29ers.

Which doesn't seem like much to me. Seems like 29ers are the way forward.

Triple is technically the best option.

I want to know if I can skip buying 650 wheels and just get a "gravel" frame that clears 29x 2.2

This is down to geometry.

I found that if you fit bigger tyres that max the frameset clearance, it has a noticeable difference in handling (putting 38mm on a Canyon Endurace), fitting a 28mm back on revert it to how it should ride.

The geometry may need further tweaking to really take advantage of running a big 29ers (such as the Salsa Cutthroat).

650b is excellent to be able to retain the handling characteristic of 700c bicycle without too much compromise, basically adding a plush plush tyres on a bike that handle best at a certain wheels circumference.

You don't need bigger rotors for 29, you might want bigger rotors if you decide you need more braking power whatever your wheel size.

I'll nerd-bite.. A 29er wheel has about 7% more OD than 650B, more stuff at the edge, it'll weigh about 7% more. The only thing that matters is the weight since the bigger wheel rotates slower at a given speed. 7% isn't enough for me to care tbh as wheels affect more important things when it comes to off-road riding.
My take on it seems to be about bump frequency and amplitude. Frequency is handled by volume, amplitude is handled by OD and roll-over. 650B is great for a road-gravel bike as it's got the volume potential at a lower weight, works well on fast, small bumps. 29ers can handle both so they make more sense for an XC MTB.
I like 650 x 50 for road and light gravel, basically anything where drop bars make sense. Another benefit to 650B as Ed said is you can get guards in and still have a fairly agile road geometry.
When the bumps get bigger and chunkier I'd go 29" but I'd also drop the drops by then, it's XC MTB time and I'd rather use an Jones bar or some sort of MTB bar than drops. With the wheel capability comes the want to handle the bike differently, for me anyway.

A 29" drop-bar like the Cutthroat could be good if you have a lot of that middle ground riding, neither road/gravel nor XC MTB. Most of my riding is more one or the other or I prefer a bike that has clear advantages and disadvantages vs something in that middle ground.

You chooses the kind of grams you value. There are grams to be saved and then there are grams to be ...grammed?

Clearly 26" is the answer. Even lighter than 27.5!

24" wait 20" wait 16" wait...

Jogging is now the ultimate way of gravel cycling!

Only if it's barefoot. Because weight.

27.5" feet though

I like how this post covers over subjects like how much is too much a tire for gravel or at what point having drops makes no sense.

Or how much tyre is too much for a drop bar? Maybe better if I'd said 'traditional/common drop bar uses' rather than where drops makes sense ie makes sense to me.
For tyres, let's assume gravel to fast XC tread and volume on either size rim. I might go a bit bigger on a 29er as I'm on that route to more technical ability but the point's valid if you assume general 650B gravel tyre or 29er XC tyre.
All I'm getting at is how these things add up and a bike can have parts that pull in a direction or gives the bike a useful bias. But if a drop-bar fat bike makes someone happy, all good and no logic needed.

I guess it depends if there's room in the shed for an xc bike and a gravel bike as well.

That said I might just be finding my thoughts drifting this way because my "gravel bike" is a 2015 croix de fer with the anaemic clearances that entails.

Its interesting to hear your thoughts on this. Obviously you’ve put some thought into it.

My take is that I really appreciate to have as little stuff as possible. I don’t want a stable of bikes, multiple wheel sets etc. I get that some do and why. Anyway, I value the fact that my bike can be almost road bike and almost xc mtb. To me this jack of all trades master of none is a good thing.

I get that, doing more with less is always good. I think I'm happiest with the two option either side as I like road/easy gravel riding as well as MTB specifically, depends on my mood.

My take is that I really appreciate to have as little stuff as possible. I don’t want a stable of bikes, multiple wheel sets etc.

That’s my main reason for my gravel bike; commuter, touring, off road (crossing mountain passes), roadie fun, etc.

It fit the bill for everything, it just doesn’t shine well when compare to a thoroughbred road bike (unless it’s the carbon Trek Checkpoint).

Ultracycling >>>>>>>>>

Too many border crossings involved.

I tried both Cervelo Aspero and 3t exploro and both feel pretty fast on road. With a 40t 1x, go figure.

Or how much tyre is too much for a drop bar?

I now have an irresistible urge to build a fat bike with drops. And aero extensions. Just for the LOLs.