I responded to your other thread before reading this one, assuming that by "shell bean" you meant "shelly".

For beans intended for dry use, yes, picking them as snaps would stimulate the plants to produce more pods. Those pods are likely to be smaller, more fibrous, or less palatable than beans bred specifically for use as snaps. I have seen posters who swear by the quality of pinto beans picked young as snaps... but personally, unless you have limited space & are looking for a "one-size-fits-all", you might be better off just planting two varieties.

If you choose to harvest some of your dry beans early for shellies, though, those plants might rebound & produce a crop of snaps. To take advantage of that, any plant harvested early should be picked bare. Those plants will (usually) put out new leaves & blossoms... then it is just a question of whether they will have time to produce before frost.

If all the above makes sense, then dry beans should be less productive than the snap pole beans.

I just wonder how the bean growers can produce dry beans at such lower price, if the production is much lower than snap beans. Maybe we'll have to pick snap beans and machines can just harvest dry beans....

Per plant yes, but can tolerate denser planting. No manual labor involved with bush beans. Everything can be done with machines. fast and/on large scale. No machine available for setting support or harvesting pole beans.