Hybrids escaping and breeding with native pythons, this does happen, there was a thread on another site where a person on the central coast of NSW (diamond python central) had been called out to remove two separate pythons, one of which was a hybrid the other was a coastal, neither of which belong living wild on the central coast.
The other problem is people wanting pure animals and collecting more from the wild putting more strain on wild populations, this will happen it already is happening.
I'm sure i could come up with more.
From a hobby point of veiw i don't like people crossing but i do realise that it's now part of the hobby and those who wish to cross breed snakes should be encourgaed to be honest abouty what they are doing rather than slamed for doing so, that way me may have some hope of keeping 'pure' and 'hybrid' pythons separate.
Your first point a very common one is completely flawed and has no relevance to hybrids in particular any snake that doesnt come from the exact area where you live can have this effect and with the amount of incompent ppl letting animals escape this is a sad reality that comes with ppl keeping herps that dont come from their area.
As for ppl collecting from the wild, they may do, in 99% of cases it wouldnt have any real effect on populations, if you think it will please gives some evidence to show how much of an effect it has. The fact that this can happen(even if illegal) destroys the arguement made by others that pure snakes will magically vanish if ppl breed hybrids.
You last point is right on the mark