The world would also have twice as much more disappointed players that are cleared and nobled in their off times if cooplaying is forbidden. People tend to quit the world and often TW when their work get blown away in the second while they were not there to take any potential defensive actions. If they're nobbled by bigger player wile account is on, well that's considered a more fair play for most especially if morale is on their side. Sure sitters may help, but I wish you good luck finding a trusted sitter on a regular basis, plus we should not forget there are sitting restrictions in play that may limit teamwork efforts.
I've played a lot of worlds under this and other names, both solo and as a Co.
Ultimately it's not hard to be one of the top 2-3 in a 13x13 playing solo when a world begins.
I can only think of one occasion where I was genuinely outclassed when playing in earnest. I am by no stretch of the imagination a world class player.
Nobody is getting rimmed while they're asleep before trains. You can't get cleared if your troops are out farming overnight. If you're playing smart your immediate neighbours won't get the chance to research cats, let alone demo your village.
While I understand being rimmed is disappointing, it is part of the learning curve for many. Frankly if nobody got the disappoint we wouldn't have much of a game.
By the way, I don't think it should be forbidden, I'm not against Co-playing. I totally get where you are coming from, I just think the shift in the culture of the playerbase has had a detrimental effect on worlds.
It's all good and well for the player base to say Inno are killing the game with pay to win mechanics, I simply think its important to acknowledge that the community and the way we chose to play is also a factor.