You said that it's almost certainly the case that people building their own crypto are doing it solely for their own purposes and that people won't even bother to complete their work. Obviously: no.
I wrote "almost certainly the same is true of crypto" meaning that almost certainly very few people actually want to implement crypto (just as very few people actually want to build airplanes) for any reason, and so the desire to do so serves as a pretty effective filter to eliminate "random people" doing it.
Of those few who want to do it, some will want to do it for personal reasons and some will want to do it for commercial reasons, just as in aviation.
