The dream was never George's anyway - it was Lennie's. It was George's way of soothing Lennie, of reassuring him that George would not leave him. George equally needed the fantasy to give him a reason to stay with Lennie. It provided a bond between them, but always had tinges of unreality about it (remember the coloured rabbits?). George's 'real' dream was epitomised in the small insight into an equal, true friendship which he experienced with Slim: a man who could really respond to him, share his burden and offer him real comfort.
George's dream with Lennie was merely a bedtime story to placate a childlike imagination. It was a story they shared like any other fairytale. The practical plans to attain the farm with Candy's money only occured after George and Lennie decide they do not want to stay at the ranch, with the threat of Curley and his 'jail bait' wife. George knows they are running out of options for work as Lennie has cost them many jobs. Sending Candy and Lennie ahead to set up the farm once they have paid the deposit is as impossible and ridiculous as 'red rabbits and blue rabbits and green rabbits.'
The dream really wasn't a dream at all without Lennie. It was kept alive through him. George always sensed that the dream wouldn't happen because of the trouble Lennie got into. However, it was the dream that kept George and Lennie together and kept them going forward. Without a such a dream, George could not have continued to keep up with Lennie because of all the trouble he caused. By hanging on to Lennie and the dream, George could continue on each day with hope that something better in life was just around the corner. Once Lennie was gone, so was George's dreams and his future. As Crooks explained, a guy can be lonely without someone to talk to. George and Lennie weren't alone because they had each other. In the absence of Lennie, the dream was dead and George was all alone. Candy and other helpful men could not have completed George's dream. It was Lennie's innocence and child-like adoration of George that completed the relationship between George and Lennie and kept the dream alive.
A great question. You're right. Other men could provide the physical and financial components that Lennie would provide, and Lennie certainly wasn't providing the intellectual guidance. So what else was missing once Lennie was gone?
The heart. George spoke to Lennie roughly, but he loved him as a friend and a brother, and he'd watched over him for years. The dream was a place where they could be free and relax, and where Lennie could be Lennie without fear. He'd miss Lennie too much, and always be reminded of what he'd done.
It's not George can't carry on his dream without Lennie, in fact, he wants to. However, like you mentioned, George can't carry as many physical work as Lennie.
But Lennie's aunt died and she is the one who took care of him the most. George felt bad and felt the huge need for George to take care of Lennie (and soon felt like Lennie was like a brother to George).
Any more answers are more than welcome please!