I question whether contenders for the world championships will do the Vuelta next year, except maybe solely as preparation for worlds.
I think Roglic proved the other day, that you cannot do the Vuelta full gas, and still be at your peak at worlds (we'll know more Sunday).
Add to that, that next year a lot of these riders, will already have hit the 2nd (Ardennes being 1st) form peak mid season, because of the Olympics.
Though you may be right in the specific case of Valverde..... he's won worlds, and because of the mentioned dynamics, next year may be his best chance ever, to win a Vuelta, so he may prioritise differently.
I would question whether, given the unique circumstances of a year where being both Olympic and World Champion on the road is a realistic goal for the punchy GC guys and hilly classics specialists, a lot of the leading contenders may choose not to specifically taper and peak for the Ardennes as they get over 10 years to race for the win there and instead just ride them as form builders and training exercises.