Sorry, I misread and I agree there isn't that much difference between P1 and D4.
But I'm not sure how much weeding out is really going to be done here. I think Respawn's original concern of people quitting is fairly spot, especially in that crucial Pi v D4 category you speak of, with a silly dive trail at stake (is it really a big deal? no, but I think people make it a big deal).
A D4 getting demoted might play on, but I think it is more likely they'll quit on the first D4 hit, or...after they have to struggle back to D4 again once, or twice, they'll quit soon after that. I guess it is weeding out in one sense because less people will be playing ranked, but is that the right solution?
This happens with all the people leaving threads too, and as I've said there, nobody has to play this game. Nobody has to play ranked either. If Respawn is totally fine with less people playing ranked, then maybe this is an okay decision. But if they are trying to promote people playing ranked, they've only included punishment and no real reward. That's not the right goal.
What reward people get for playing ranked is something up for debate. I think people played ranked because they were under the impression that it was better because less people quit and there was more of a chance of them playing together. These benefits might be tossed if players are more concerned about their vanity...and frankly, I tend to think most people are.
This is not going to affect people who play ranked just because they like it, and are willing to put up with demotion risk.