There was an incident in season 37 where a contestant playing Plinko was given $30,000 because they didn't remove wire that set up the chip to hit the $10,000 after filming a promo for a video game-- what if this had not been noticed and the contestant won $50,000 as a result? Would they have given the contestant $50,000 and then reshot the game to be played honestly like they did, or would they have kept in the tainted $50,000 win just to say someone won Plinko and get publicity out of it? They didn't use the perfect showcase bid for publicity because they assumed there was cheating involved and finding out the circumstances behind the Plinko win certainly would've been a PR nightmare.
As for the possibility of Plinko being fully won without anything like the above incident happening, probably the only way it happens is if they change the board to have two $10,000 slots.