That’s completely impractical. You can’t erect crowd barriers on a 2000+ mile route of public roads. Roadside crowds have been part of the TdF for the last 100+ years. There’s a rolling police motorbike escort which moves in front of the peloton to ensure spectators aren’t obstructing the road.
Then expect more adults riding bicycles to have more slap stick pileups....It’ll be good for the ratings since it’s the only exciting thing about watching that silly stuff.