right, but if you have a road casette, and a 36t chainring, you have the same 'top' gear as a mtb casette and a 36t chainring.
The big chainring era allowed cranking at higher speeds. This was more because of the types of DH trails back 10 years ago were generally faster and smoother, so ridden at higher speed.
road cassettes have advantages. Closer ratio gears allow the rider to be in a gear that is more 'right' for the speed they are doing. they also allow a shorter rear mech with less slack chain. However, if you're not good enough to get increased performance from having your gears ever-so-slightly closer together, then you may as well run a mtb cassette (I'm rarely in the 'optimum' gear when riding dh, i tend to bung it in one gear and crank away).
Smaller front chainrings are an advantage. They are *not* an advantage of road casettes