The other 78% moved into another high labor demand field, manufacturing. When this happen, there was a real strain on the agriculture industry because of the labor shortage it created. Much of the mechanization in agriculture was out of neccessity, not just for profit. There is no labor shortage in manufacturing driving the mechanization. Labor is being pushed out rather than pulled out by something else.
We are reaching a state in our society where there simply is not enough work for everyone to maintain a middle class livelihood. Maintaining the status of the middle class will require a radical shift where the value of jobs currently seen as low value (such as a store clerk) goes up. You need to be able to make a real career at anything. Different examples of this can be seen throughout the world (such as the skilled fastfood workers in Japan), but they have not all come together under one society yet. Huge leaps in the minimum wage would be a good start, but the problem is a lot more complex than that. Changes in how education is accessed, single payer healthcare, and fundamental changes in the world's monetary systems are inevitably needed. There is no longer the scarcity of resources that once existed, and much of the current scarcity is artificial to keep profitability. This is stupid. People right here in the United States go hungry, but we have more than enough food to make everyone fat.