At first I was thinking like you - Vuelta win is more important. However, while I agree that a Vuelta win will do more to his palmares than a WC ITT, TdF + WC double is something you're unlikely to see in the next 30(?) years.
As hard as TdF+Vuelta double is, Indurain, Contador, Froome and Vingegaard (I mean..even Roglic was 1 minute away from doing it) were capable of doing it and then you can add every cycling great from the past. I have no doubts that the next dominant GT rider (being Pablo Torres, Jarno Widar, Jørgen Nordhagen or some name we haven't heard yet) will be able to do it as well.
TdF + WC double (+Lombardia) is a feat only the true GOATs (as you put Merckx and Hinault) can achieve.