At the top level - which is pretty much the definition of a tour winner - grams make the difference between a win and loss, so of course they will not be riding steel.
I'm not at top level and never will be. If I want to go faster there are many things I can do that are more effective than playing what my bike is made out of.
You can make a steel frame bike which will be lighter than the UCI weight limit with modern technology. It's just nobody cares to make one because the "unracer" crowd who believe steel frames are magic, won't be spending a couple grand on a light steel frameset and the actual racers are fine with the carbon frames, where it's much easier to control properties of different parts of the frame through fiber layout.
Non-racers appreciate light and comfortable bikes too (if anything it's much easier to load a 20 pounds bike on a car/bus rack than a 30 pounds one), this is why carbon is now even in the midrange consumer segment. And with the tubeless tires even on the road bikes nowadays, the magical suspension properties of a steel frame are hardly noticeable over the effect of lower pressure in the tire. So I don't think steel frames are ever coming back.
That's not really true, if you can make a ultralight full-featured steel-framed bike there will be buyers. There is a not so small crowd of "weight weenies" and/or steel frame fans willing to spend big.
How do you know? Do you believe weight weenies are only into steel and this is why they don't buy ultralight carbon bikes (there are few under 3kg carbon/composite bikes that had been made but nobody bothers with industrial production of 100K bikes somehow).
Somewhat. UCI has a minumum weight (6.8kg) for bikes and often teams have to add weights to their bikes to hit the minimum limit. You easily get lighter bikes but your bike needs to be 6.8kg to be race legal. It certainly limits how many bits and bobs on the bike get switched over to using titanium or carbon fiber.
I'm not that deep into racing, but I'll guess that teams still want to be way under that weight so they can strategically place weight where they want it.
They may want it, but the technology only recently converged to the point where you can have an aerodynamically optimized bike under the limit weight. Before that there were special lightweight ones for the mountain stages and heavier but aerodynamically faster ones for the flatter stages. Most teams still got these two, plus of course another bike for time trial, because time trial stages are not mass start and the bikes and rider positions can be even faster and less safe there.
