I'm not sure (in this case) it was about Python being established and able to do what they want, but more that this sort of extended line suited Cleese's style, plus he could perform it to the others (who made the script decisions rather than an editor/producer) to prove that it would work.
Very few performers (or sitcom characters) can make a really long line work and the chances are they aren't working on the shows we're submitting to...which means long lines will just suck life from the sketch/sitcom. So script editors/producers will (rightly most of the time) just say "too long"