Would I be right in thinking that your reasoning would say that if that had happened, and plate tectonic mechanisms had not intervened, we would now have an Earth that was completely covered with one type of crust?
I would think that earth already has 1 type of crust.
like fresh water is basicaly the same as sea water.
the earths crust is also basicaly the same as far as chemical composition is concerned , it is the difference in the amounts of chemicals found in basalt and granite that cause the two to seem really different.
an active basalt flow
granite collums
they are both igneous volcanic rock , or cooled lava
the earth has two types of crust
basalt and granite and both types are igneous rock
and plate tectonic mechanisms had not intervened
my guess is that if that would have happened the entire forming mass would be too hot to remain solid , even the two cores would form a single core , after several million years of cooling then the crust would be taking shape , remelting , taking shape , etc.. for a few million more years.
Plate tectonics would have come about during the cooling process.