Cause, no, but definitely the turning point in the tide. Before and during the nationalist fervor of the war, there was a bit of a movement to purge Japanese of foreign loans (敵性語 "enemy language"), similar to sauerkraut turning into "liberty cabbage" etc in the US. Once the war ended, this was swiftly reversed and the floodgates to importing foreign terminology wholesale (re)opened.
China wasn't an "enemy", the Japanese had already conquered large swathes of it. The US and Britain were.
And yes, you can write "pure" yamatokotoba if you try hard enough (see eg. Shinto prayers), but the end result is as contrived as trying to write English without Latin, Greek or French loans.
Uncleftish Beholding (1989) is a short text written by Poul Anderson. It is written using almost exclusively words of Germanic origin, and was intended to illustrate what the English language might look like if it had not received its considerable number of loanwords from other languages, particularly Latin, Greek and French.
https://ja.wikipedia.org/wiki/敵性語