That's just terrible reasoning. "Oh, they might have, maybe, become disengaged anyway, so let's disengage them as much as possible now." Even if that's true, which it isn't, this was an especially important issue for a lot of people who just felt the need to be represented and have their voice heard, but even if it was and they disengaged a year or two years down the line, you still lost a year or two years of activism and small donations and community organizing. That has a big impact on your 2018 chances.
Let's face it, there was no real reason for Perez to win this. Anyone going "oh, well, eh? doesn't really matter" is basically just a wounded Clintonite holding a grudge against the Sanders wing for whatever reason not wanting to admit it. There's no connection between "they're exactly the same" and "Perez should win". If it didn't matter to you, but you knew Ellison mattered to other people, then you'd have gone with Ellison. The fact you didn't, and that you can pretend not to care, means that you actually did care, and just didn't have the guts or honesty to admit it and own it.