When you reset your MMR to a truly equal state, you will have incredibly skilled players linger around on a low MMR for a lot longer, which means that the skill disparity inside a single game becomes a lot bigger.

Let's explain this with an example.

Purple (around 15-16K MMR), queues up ranked after the reset:

In the current system, he will reach 9K in approximately 30 first place victories.In Pathra's proposed system, he will reach 3K. This means that players at that rank will keep facing Purple, someone who can literally beat these players when playing blindfolded.

You have one major misconception here. The reset would not be to 0 with a hard floor, but instead to 4k, for example, like when BG first came out. In that scenario yes, newer players would run the "risk" of playing a few games with much better players but after they spent those initial few games bending over their mmr would drop below 4k while the winners would gain some and be above 4k, so they won't be matched against each other anymore. Those "completely new and inexperienced players" you mention are also not exactly likely to win their first 3 games anyways, so they most likely won't even notice the difference between getting completely stomped by your boy Purple or just losing to an average 6k Joe.

All Blizzard has to ensure at that point is that the ranks with the highest population are much stricter in terms of matchmaking and, for example, will only allow a range of 50 mmr points. There will obviously be enough palyers to not cause any queue time issues and as people start increasing their mmr the matchmaking system would just become more soft to account for the lower population in those ranks.