Here's a question for those in the know. To what extent do the host towns of the first week subsidize the cost of the mountain stages without a proper finish in town? Meaning, the route designers know what makes a proper race, but they are also there to raise cash from the bidding process to pay for much of the race. (Am I mistaken in this at all? Please correct me).
The solution to me isn't about getting the tour organizers to realize flat stages are boring racing, but getting them to realize that an exciting stage is better for the ASO business than a flat stage. TV ratings and viewer attrition aren't part of that equation (TV ratings are hard to measure, it is the broadcaster that cares about viewership primarily).
So in that lens, a combative classification, something they could sell sponsorship for, is one solution, for example (probably as a team classification, something that wouldn't need a jersey). Or if you want a hilly stage, get companies to sponsor the climbs so ASO is more interested in more of them ("The Mur de Bretagne, catagory 3 climb brought to you by Skoda"). Or if you don't want to sell out that much, let Skoda plaster their ads all over the pavement and boards (which I know is already done) to a high degree. Or make a minor prize for fastest ascents of each climb. Sell the official start of the climb as a thing for sponsors to buy ("Tissot presents our official start of the Mur de Bretange"), sell the prize name and sell the podium ceremony background poster space. Something to make the ASO more intere$ted in a course with more features.
To incentivize teams to get in a break, maybe change the prize purse. Double the prize if it is won by the day's breakaway. Or double WT points for a breakaway win.
Overall, the route itself isn't the root of the problem. It is that the ASO doesn't care if it is boring.