I think that's far too simplistic.
We know Stones has had a lot of things change. He sorted out his personal life, he got told he was in last chance saloon at the club, he hit the gym and bulked up when he was out of the side.
Dias has given him the stable partner he probably needed to hit the heights he has done, but if you put 2019 John Stones next to Ruben Dias he'd not last 3 matches before being replaced by Laporte.
I agree with this but I don't understand how the OP could look at this season so far and say that Stones, not Dias, is the undroppable one. Dias is the leader, Stones the partner. That's not to say they're not both excellent and essential, but I've seen several people now suggest Stones is the better of the two and I just don't see that myself.