Wouldn’t even be logical to have that rule:
Say Hamilton blow up an engine at the sixth race. The engine should last 7. The planned upgraded engine is ready to be put in because it was planned as the second engine to tun after the 7th race. But since you made up a rule that everyone needs to run the same spec, he must use a spec one.
One race later the rest of the MB drivers get the planned spec 2, and therefore Hamilton must change as well after one race because you want the same spec for everyone.
And Hamilton is forced to start at the back in race 8 and abandon an perfectly good engine which is used only once.
See how silly this is?
Not only that.
This would escalate costs for the customers.
Renault's spec C required the car to be modified so that the new shape engine would fit. RBR was prepared to spend this money to fit it in. Renault and McLaren clearly didn't want to go down this route. According to pdac's rules the two other teams would have been forced to modify their cars to fit the spec C. Not an existential problem for Renault and McLaren perhaps but it could be for a low budget team (such as Force India mid-season before the buyout). Honda's never deployed final spec in 2017 also required McLaren to modify its car to fit which they weren't prepared to do so this is a more common issue than you might think.
Also PU contracts often stipulate how many units the team has access to. Additional units on top (as long as the failures weren't due to manufacturer error) cost a substantial amount extra which is why customers often husband their engines extremely carefully and are prepared to run them in detuned form to maximise mileage. According to pdac's rules an inconveniently timed engine failure could result in one or two extra engines having to be purchased, again significantly increasing costs.
The rules were never intended to have all cars running the same supplier engine to have the same spec. They were introduced to improve reliability and limit the scope for upgrades with each one with the since abandoned token system.
Edited by Talisman, 18 January 2019 - 14:05.