There's a pricing strategy used for some products called a "loss leader," the idea being that a product is sold at a loss in order to entice customers into a store or a platform so that they'll spend money on more profitable items. Solo play being an equally viable way to play the game would...
Incentivizing group play by limiting solo play seems like a very archaic approach to game design. And with a smaller game population it's just plain silly. From what I've seen if people have a group available to play with they're going to choose to play with a group over any minmax efficiency...