A big part of the problem is the "YOU CANT TELL ME WHAT TO DOOOOOOOOOOO" factor. That's not going to change. No words; however eloquent they may be will change that. The average motherfucker on the internet is not there to take orders. In fact, if someone were to tell them in a fashion that rubbed them the wrong way "Go left, god you fucks are so stupid this is why we lose we need a gfalidsjklcjreas" they're going to do the complete opposite thing to spite you. People just don't give a fuck, man. Nobody seriously thinks that "LOL WE GOTTA WIN BY CHARGING THEIR 10 RANGED OVER THERE". Maybe there's a couple, but they're ignorable retards. Likewise, nobody is ignorant enough to not realize that a completely organized team of 40 dudes, with poles and throwers to cut down flanking cav and a gaggle of ranged to fill advancing enemies with holes is better. The problem lies in the fact that THEY JUST DON'T GIVE A FUCK.
A new server for teamwork is not an option that would fix anything. I don't go on siege, so I'm always on community, NA1, or duel. Except for rare occasions where it gets maybe 20 people on it or when NA1 is down, nobody is on community. People are going to join the fullest server that there is. That's how it is, it's how people do. You'd be lucky to get 20 people on that server maximum, and organized tactics in a server population that small seems silly.
The only suggestion that I can give you people that seem to be terribly upset by the fact that random people on the internet are NOT going to act in the interest of the entire team is this: be fucking polite and personable when you're telling people what to do. Whining that they WON'T do what you say, and saying THAT is the reason the team is losing is going to make them spite you more. Why the fuck would I suddenly listen to someone after 2 rounds of ineffective orders now that they're being downright rude and whiny? I fucking wouldn't, and I would do the opposite thing because I'd be damn sure that would be more fun and humorous than listening to the braying jackass field-marshall.
Be NICE. Say please every now and again, and not in a pompous manner. People have to generally like you to do what you say on the internet. If people don't like you, then you're shit out of luck if you want to be the field marshall. Let me throw a scenario out here.
Robert Namo of Astralis is a very good player, and he's obviously played this mod for a long time. People generally have a positive image of him, it seems. NA1 had 80 people in it one night, Namo gets on and types in teamchat the simple instruction "GO LEFT" about 8 times in a row. The entire team went left and we slaughtered everyone with about 20 players to spare on a 40 man team. Namo did this once more, got teamswapped, and ordered the other team around with simple directional commands. THEY slaughtered us in a horrific fashion. After namo left our team, I tried to take his place. Not many people know me too well, and I am certainly no Namo. I'm not better than most of the players on the team, so while some of the team listened, some didn't. That's fine, I can't command an entire team after playing this mod for 2 generations. I don't mind that at all. I certainly didn't blame anybody, or act rudely.
Now, let me throw another one out there. It was NA1 with an 80 man server on that one map with the 5 sets of stonehenge looking ruins and lots of field in between them. And some hills, too. The good Reinhardt here ordered the team to set up a shield wall, in what I (and most of the team apparently) felt was a self-important and pompous fashion. Some of us set up a wall (including me, I know thats a solid way to win that map easily in pub), most of the team charged forward and got fucked hard. Next round, reinhardt bitched a little bit, and did the same thing. Same result. The round after that he was releasing a torrent of insults to the team and to players on it. Guess how many people listening to him and held position on top of that hill? A lot fewer. I know I ran forward just to make the motherfucker more upset. I'm not going to listen to somebody that is genuinely insulting me and my teammates. Thats how most people are. We only lasted one more round (it was obviously an 0-4 map) and by this round just about nobody held on top of that hill, we got fucked harder than any other round. It was butt-devastation. Senor Reinhardt continued his tirade, and there was not an ounce of peace to be had in the land.
tldr; If you want people to listen to you, don't be a jackass. Be polite, be friendly, be personable, and have a good reputation. If you don't have these things/don't act in an appropriate fashion, people will not listen to you, and will do the opposite of what you say. You'll fuck the team worse than if nobody used chat during the map one time.
Edit: Just read back over what I typed. I mean no insult from any of it. I've got nothing to gain from talking shit to a random person on the internet. I probably caught reinhardt in a bad moment, and he probably doesn't go around trying to field-marshall ineffectively all the time. I've only seen him at his worst.