I think the materials are close to the limit
No way near.
A lot of brands who were/are pushing the limits of technology simply can't justify putting them into mainstream production as the volumes are so tiny (few hundred bikes maybe) and the variation in user profile is so massive (anything from a 10% body fat, glide over everything team riding race whippet to an overweight Audi driving millionaire, who thinks riding into rocks (instead of over them) is what MTBing is all about).
And they are competing with bikes that are *nearly* as light, *nearly* as strong and half the price.
Whats the point of selling a sub 700 gram XC race frame (which would be fairly easy) when one of the first things that's going to happen is some great lump will buy one, bling it up, get rattled to pieces, put a 140mm fork and suspension seatpost on it to "improve" the ride, then send it back under warranty (or sues) when he rips the head tube off when he's crashing down a local downhill trail. I reckon that'd take about 3 months.
XTR has changed too, it used to be a lightweight, "sponsored racers only" groupset, but because its lighter, everyone wants a piece of the action. So generation on generation, the groupset has to get heavier (or stay the same weight) to allow for Joe Public using an XTR rear mech and chainset on his 150mm AM rig. I bet, even tho they now have a trail version, some will STILL put the race version on their 160mm AM rig (160 is FAR better than 150, didn't you know?) to save a hundred grams, and for bragging rights.
TBH, given a tightly controlled design spec, a tightly controlled pool of riders to sell to (pros?), and enough money (small volume innit) you could easily have a full suss enduro/trail type bike which would be knocking on the door of the UCI weight limit. Or a HT down in the sub 6 kilos range. MMCs, nano-composites, 80 kilos weight limit, minimal safety margins, ordinary composites, tiny discs, no gear ratios under 1:1 available
, 80 kilos weight limit............ it'd probably need replacing every race season , much like some of the super lightweight bikes of the 80s (early aluminium could be light, but by god it wore out quick!)
You just need to look at weight weenies to see what can be done with enough time and money (and patience) and even they are still limited to a degree, as the base parts they buy to modify are still designed with the crashing thro everything, unfit, heavy lump in mind.
(And i tried a Zaskar, way back when, Judy SLs, XTR and so on, and i thought it was crap, still don't get the love for it. Xizang was another thing altogether. But 3 times the price!)