It's not as good a field. But then, no top team HAS to go to either. But some still do. Nibali, Pozzato, Kolobnev, Rasmussen, Jackson Rodríguez, Francesco Chicchi, Serpa, Botero, Peña, Kiserlovski, Napolitano, Cheula, Valls, Gómez Marchante, Niemiec, Baliani and Ochoa all lined up in San Luís... not a bad lineup.
In the TDU, all the top teams MUST attend. It's no wonder that it's a bit skewed.
What I'm saying is, making a race that is tougher than the TDU currently is will not stop people attending. They won't attend if they make it really difficult, but San Luís is harder than the TDU is and still gets a decent field. Add the ProTour points and the organisational nous of the TDU guys and you'll still get a good field.
I also said, they don't need to make the race harder even - they just need to place some of the hills they already have near enough to the finish to open up an option, just the slimmest chance, for anything more than a bunch sprint. If you tempt people to form small groups and try to break up the sprint, then they'll do it, and you get unpredictability. If it still ends up in a bunch sprint, then so be it, but we'll have got there by a more interesting route than the race currently is at.