Hi Happy Feet,
It can be very difficult so separate marketing hype where even the most modest change is often sold as a major breakthrough, from real-world functional advances that have measurable benefits. But you are right to you conclude that some developments in wheel and tire technology have produced real advances in both MTB performance and capability.
However some of the benefits are very terrain specific. For instance, in the UK we have a variety of off-road cycling traditions one of which is called Roughstuff riding where traditionally, unmodified road bikes are ridden off-road. About ten years ago I took a Giant NRS full suspension MTB on such a ride. Because the NRS had a good reputation as an XT race bike I thought it unlikely that anyone would be able to overtake me. However on a long steep off-road climb I was overtaken by riders riding 1980s road touring bikes. They rode out of the saddle whilst using what looked like surprisingly high gears and where obviously used to tackling hills this way. At the top of the hill the track was bumpy so I soon caught up and going down the other side I was able to ride down at high speed whilst they came down slowly, braking in order to keep control of their bikes. In the early years of UK MTB racing Cyclocross riders used to compete and win races. The organisers responded by modifying the courses to make them more difficult for the Cyclocross bikes. There is no one design of bicycle that is ideal for all kinds riding conditions. General purpose bikes are inherently compromises, Jack's of all trades, masters of none.
Another interesting problem is the best width of tire for riding through mud. If the mud is only a layer on the surface then a narrow tire can be best, as it will efficiently slice through the muddy surface to find the firmer surface below. In such circumstances a fat tyre will be less efficient as it has to push much more mud out of the way. However, with deep viscous mud the fatter the tire the bettera apart from situation where the mud sticks to the tire and gums everything up.
The Finnish snow riders had a similar problem. On firm compacted snow a well treaded tire of any width will work, though a large diameter wider tire will have a larger contact patch and so more traction. However on soft snow there is the option of using a narrow tire to efficiently cut through and find the firm surface below, or a fat tyre in the hope that it will sink in less. The Finns used both narrow and wide tires that could also be fitted with tungsten carbide studs for riding on sheet ice. The rubber compound was engineered to remain supple down to -40 degrees Celsius.
There appear to have been two distinct Finnish traditions, a civilian one based on riding on snowy icy roads, and a military one based on all-terrain riding. The bikes I have seen are all single speed so it was probably a case of get off and push if the going gets tough. The Finns has regiments of troops who used skis in the winter and bicycles when there wasn't enough snow for skiing to be practical. These regiments had a long and successful history including holding off the Germans in WWII and various Soviet invasions. The 1970s/80s military bikes were fitted with fat 650B tires.
Early British 700C off-road bicycles had reinforced frames and highly effective hub brakes. Even so, they suffered from wheel breakages caused by poor-grade aluminium rims and hub-flanges that were too narrow. In 1985 an engineer called David Wrath-Sharman started making these bikes and by 1987 he had solved those structural issues. This involved manufacturing his own hub brakes and other components whenever suitable alternatives were not available . In the late 1980s he went on to develop suspension systems for his bikes.