Also, just to be clear, i am just as excited as you are about the DemiLigt partnership taking shape but i also believe that for now experience is still needed, it's more likely that next season will really be their breakthrough one with both featuring more often as regular starters, regardless of Chiellini renewing or not, they are the future and have plenty of prime years ahead of them.
Tbh, i would give more credibility to your narrative about making place for the new guys if circumstances would have been different but if you really want to look at it with a clear head you will realise that both DeLigt and Demiral had enough injuries since being with us to leave us exposed at the back and haven't had as many games when both were available and fully fit and if it wasn't for the old guard still being around and also with a bit of luck it happened that usually when one got injured the other returned and viceversa that overall makes us more solid and on the bright side the young guns have only to learn from guys like Chiellini and even Bonucci.
Don't know how well i've explained it but IMO it's definitely better to have all of them around, especially for another season.