When I said "solid bitumen", I was referring to traditional road materials, and not a bitumen only tarpit. Sorry for not being specific.
Also, "durable" is a relative term. We're talking about roads. Solar panels are durable when compared to, say, laptop screens. They are not durable in the context of road surfaces. Yes, there are amazing glass types around today, but once again, in the context of road surfaces, I don't think glass is, or could ever be, an appropriate material.
Bitumen+gravel is used because the stone gravel provides excellent wear resistance while the bitumen holds it in a flexible and self-healing suspension. It is still the best road surface material we have by a country mile.