Why would it be much worse for the community to release Jonas instead of let him run away in Lois Lowry's The Giver?

If Jonas was released, the memories would all immediately flow back into the community. When he runs away, the memories trickle in as he gets farther away. We can tell this because Jonas has increasing difficulty accessing memories the farther he gets from the community. 

The Giver explains to Jonas that the memories a Receiver has will return to the community if the Receiver is lost. 

The Giver was quiet for a moment, obviously thinking. "You know," he said, finally, "if they lost you, with all the training you've had now, they'd have all those memories again themselves" (Chapter 18).

When Rosemary committed suicide, the community had to receive her memories. They were almost all positive, but it was still a very confusing and difficult time for them. The Giver was there to help them through it, though, because Rosemary was only in training. The Giver tells Jonas that he has many more memories, so things would be much more difficult for the community if he was to die. This is why the Giver tells Jonas to stay away from the river. 

After Jonas finds out what release means from watching the video of his father release the newborn twin, he and The Giver make a plan for Jonas to escape and for The Giver to help the community with the memories they will receive. 

If I go with you, and together we take away all their protection from the memories, Jonas, the community will be left with no one to help them. They'll be thrown into chaos. They'll destroy themselves (Chapter 20). 

Jonas and The Giver develop a complicated plan for Jonas to escape on Ceremony Day, when no one will look as hard for him. By the time Jonas is away, The Giver will be able to address the entire community and help them “change and become whole.” 

Their attention would turn to the overwhelming task of bearing the memories themselves. The Giver would help them (Chapter 20). 

Jonas has to flee early when he finds out Gabe is about to be released. He takes Gabe with him and leaves without saying good-bye. He knows if he is found he will have broken enough rules to be released, so it is very important that they make it away from the community. 

Since Jonas has to leave unexpectedly, he did not get all the memories of strength from The Giver. Also, the farther he gets from the community, the less he can rely on the memories he has because they are being returned to the community. 

Sometimes, urging the memories into Gabriel, Jonas felt that they were more shallow, a little weaker than they had been. It was what he had hoped, and what he and The Giver had planned: that as he moved away from the community, he would shed the memories and leave them behind for the people (Chapter 21).

The escape is successful in that Jonas and Gabe get away.  They have to go through rougher and rougher terrain, and finally reach Elsewhere. Since the ending of the book is ambiguous, it is unclear whether they pass out and die or are rescued. Either way, they are no longer in the community and the memories are back with the people.