Re:
teh stainless steel alloys are pretty much the last innovation that steel had, probably main diff now is everyone knows how to air harden cromo so brands like voodoo and kona have their own stuff instead of reynolds or columbus etc. i think really 953 and spirit cant be considered in comparrison to cromo variants cos its hard to produce, hard to weld, and super expensive. its pretty much a slightly more exclusive alternative to titanium imo
what really changed in teh 90s was cheapo chinese aluminium taking over and carbon fibre getting sorted, so if you wanted a cheap frame you got aluminum and if you wanted a light frame you got carbon, or scandium for a couple of years. decent steel more or less got squeezed out when it came to r&d money, gets to the stage where its not worth investing in steel alloys. end of the day genric 4130 is as good as anything, all teh rest is diminishing returns compared to using different materials.
then theres teh commercial/branding stuff, bike manufacturers needing something to put on the sales advert, used to be the latest trick steel, and all the lower models in the range needing slightly crapper steel. by the end of the 90s you had endless varieties of aluminium, different carbon techniques, not to mention suspension designs to tempt/delude customers with
these days from what ive noticed manufacturers seem to have all agreed that steel mtb frames should weigh about 2.2kg, no matter what steel they are made of. maybe cos of that eu regulation, even though it doesnt apply to air hardened steel. if you have a steel like 853, you could make a frame that weighed 1.6kg, so all the extra metal is there to make it stronger and more ridgid. i find they tend to feel more planted, less springy but still compliant in a way carbon and ali could never be.
i guess partly its in respose to teh way bikes have changed in general, like the way ppl ride them, in the saddle with massive front shocks and just plough through the bumps, kind of dictates a more solid frame. if you want a lightweight xc machine you would normally buy carbon now. or maybe ppl just dont like the springy feel anymore. but all in all there has been no improvement since the 90s
i have an interesting frame, its made of columbus zonal, latest version of max. the guy i bought it off had it custom made, specced it with whyte 17 geometry, which is a freeride 26er for 120mm+ forks. this frame weighs 1.7kg compared to the whytes like 2.5kg. i just cant think of any situation a 1.7kg steel frame would need or could use 120mm of travel.