Thing is, both Froome and Bernal's accidents - on the surface at least - look like cases of rider error. TT bikes are harder to handle, slower to brake, catch the wind more, and tend to give worse visibility, all while travelling faster for the same effort; everyone knows these things. But on any given summer sunday around Wales & England there will be at least three or four timetrial events, each with 120 complete amateurs, hobbyists to varying degrees of seriousness, who will manage to ride these aerodynamic contraptions without incident on roads completely open to traffic. There's no safety reason stopping the best professional riders in the world from being able to do the same. Find some quiet roads with good visibility and go train on them, these guys have more free time to work things out on google maps than the rest of us. For reasons of levelling things up between teams I can see an argument although I'm sure the big €€€ teams will quickly enough find a way to get round it.
On gravel etc... for me there needs to be some natural logic to it. Road racing at its origins is quite a natural sport, we race from place-to-place by what was the fastest human-powered method at the time we started doing it, by the fastest routes that are naturally available to us, which are roads. It's an elegance that's missing from mere games like football or tennis where the outcome can be determined by how wide or high someone has arbitrarily decided the goals or net should be. If I go to ride round the small villages of Tuscany then it's logical that I'll encounter the white roads, just as in Belgium I'll find those concrete roads with unsealed movement joints just the right size for getting tyres trapped in, or round here the hedges are so high you can't see round corners. Maybe you can make the same case for dirt roads on visits around the wineries of the Touraine, I genuinely don't know, but streetview certainly doesn't seem to find them significant enough to cover the way it does in the Sienna area. Maybe it's not actually any more artificial than the cobbles that are carefully preserved and curated by Les Amis de Paris-Roubaix, or the unfinished military road through Piedmont that magically turns to smooth tarmac just as we cross the summit and that has been rolled smoothly enough in advance for Contador to ride out of the saddle round its hairpin bends, grey areas maybe. But interest in the true gimmicks like Super Planche will quickly burn out.