I mean yeh that is true. Like not everything has to be replaced in an airsoft replicas. Most AEGs just need a reshim, relube, a new hop up rubber and nub and you have a pretty good replica allready without spending much. Stock parts are often good enough for a basic assault rifle.
If you are pushing the max of replicas tho like BASR and DMR you do need to spend a bit more. Mainly time but also money.