We already saw how Asgreen's ITT had deteriorated at the end of last season. I don't expect that to be much different if he goes from the Tour (where he'll be a hard worker) without some rest. They're not going to let him leave after 10 days. Same goes for van Aert.
Considering these riders need a few days at the very least to prepare and rest, i can't realistically see any team letting them go a week in advance. Not even Küng. Especially not considering the two biggest favorites will be able to prepare in depth. What's the use in quitting the Tour to finish 4 minutes down to Dennis?
This could only be of use in case van Aert, Asgreen, Roglic, Küng, Dumoulin... crash in the first week without real injuries but bad enough to throw in the towel.