IMHO, You are both right and wrong.

I think its more to do with the fact that as a species, we detest change. Change is never easy.

Just ask any smoker who has tried to quit.

I am not saying it can’t be done, but it sure does take effort. As far as all the hate goes, yeah — I would agree that it is meaningless. But, I have a feeling that all this hatred is not coming from the support for the 2 space approach many follow. Let me try to take a jab at the reasons behind the hate:

I don’t think many (if any) are doing it consciously

1. The 2 space habit has been there with this lot for years now. Imagine hundreds of thousands of instances where the 2 spaces have already been placed for each and every one of these guys. It has become more like a muscle memory for them. They just can’t help it. They aren’t even aware they are doing it when they are doing it.
   (And yes, it is a habit. I very much doubt anyone who has been doing this for years still does it with a conscious thought)
2. The way our brain works — Yes. As usual, the brain is going to get blamed here. Whenever we are involved in any task that requires us to exercise our cognitive abilities, that is where most of our attention span is. Everything else that we are doing in that moment — just happens. I don’t know how the brain handles it, but by now I have enough trust in it to know that it will handle it all. Whether you are writing a mail, a book or anything important, that is where your whole attention is. Take this comment of mine, for example. All I am immersed in — (1) Writing what is going on in my head, (2) Structuring what word will come after the current ones, (3) Reviewing everything that is appearing on the screen.
   So yeah, the words are just appearing. I don’t have to make the conscious decision of moving my fingers on the keyboard. It just happens on its own. The brain is relaying information based on which my fingers move, and the pattern in which they move is being dictated by my muscle memory. A whatsapp notification popped up (for whatsapp web), I got distracted for just half a second. The words stopped for 2. Imagine if one were to get distracted every single time that double tap happened on the keyboard.
3. It is more or less a harmless mistake — if I may take the liberty of calling it a mistake. Does it hurt your eyes? My eyes? Yes it does. But relatively speaking, it is harmless. So, maybe they never bothered to change it — because well, better things to do.
4. People have been asking them for a while now — People like you and I do exist. A lot. We will point it out to them, try to get them to change their habit. Maybe some of them have even tried and failed (some would have succeeded as well). So they gave up. Because, (a) Change is difficult, (b) It was actually hampering their productivity, which was further adding up to the frustration, (c.) They thought it doesn’t matter all that much, (d) Taking up any new habit that requires discipline is a demanding and challenging task. Just think of the countless people who can’t stick to a gym routine.

By now, they are sick of it, frustrated, and lashing out. It has gotten to a point where it infuriates them that this one simple and what should be insignificant thing keeps on popping up again and again.

So No! I don’t agree with your premise that people really love double spaces. I think they just don’t care and are sick of having to defend their position on this again and again.

So why do they actually provide arguments supporting the double space lifestyle?

Basic human psychology. Some of them tried to change, couldn’t, gave up, and now they are defensive about it. They have concocted a whole case in favor of the 2-space rule.

Those who didn’t try because it just didn’t feel worth the effort, they got tired of getting pulled up in a debate on the topic every now and then and having no arguments in favor of their case. Because if they couldn’t answer the question “Why do you think we even need double space?”, the other person would say “So why don’t you change?” Easier said than done, mate.

So even they have a defence now.

Could I have it all ass-backwards here? Could be. But in my experience, psychology always plays a major role when it comes to matter that look so trivial on the face of it.

A good way to fix it is how Ev Williams did it. The Medium editor itself doesn’t allow putting two spaces. Good softwares (and systems) are supposed to adapt to (and be designed based on) human behavior. Not the other way round. So if you want to blame anyone, blame the softwares who have failed to evolve. As far as it hurting your eyes goes, tough mate! I cringe when people don’t start their messages with a capitalised letter — be it on desktop versions of slack, whatsapp or anything. I just don’t like it. But that’s my problem, isn’t it?