We need a solo mode simply because playing this game with two randoms is essentially like playing solo. Either they drop separately or run off and do their own thing or rush into a fight without telling their teammates. And in all of these cases, when they get dropped and thirsted, they quit.
In a vast majority of my games – all of which have been with randoms – the other two players are unreliable. And irony here is that my teammates are probably thinking the same of me, because I am not following them or taking too long to loot.
Solo mode will take some of the people who want to play that way OUT of the current mode queue, immediately improving life for both groups. Granted it isn't going to solve the entire issue, but it should help. And it might help bring back some folks who have given up on the game because of how painful grouping with randoms can be.