Scenario for you, I don't have very much money and I never fly. I've just spent the last year saving my pennies and I finally have enough money to purchase a holiday for me and my family. I get on board and find that the seat is just a little bit more squishy than I was expecting. It wasn't malicious, it wasn't trying to game the system in hope for a free upgrade, I just honestly didn't know the size of the seats and I don't have a lot of money, certainly not enough to effectively pay twice for the one seat. Now what?
You need to remember, beyond money being tight for some people, whilst I and most of the people on this board their last flight wasn't that long ago, and they don't have long to wait until their next flight, the average person fly's a lot less than that. It's entirely possible that you'd have someone whom has either never flown, or it's been many years since their last flight who would not really have an idea just how small Y seats really are.
Long story short, I really don't have a problem with an airline putting someone whom is a bit taller, or wider than the average pax into more comfortable seating. I don't see why if someone has to be upgraded in a row of 3 that it's the COS needs to be punished because it would be the other two pax whom are "missing out on what they paid for and therefore deserve the upgrade".