All everything 'should' be free. At least, that which is not scarce. The correct question to ask is 'can' all research papers be free - does the world continue to spin, will research still happen, will we still progress, if they are free? The only reason we even have this debate to begin with is because the producers of this information require scarce/controlled resources in order to survive.
What happened here is that jerks hijacked the academic publishing industry. They turned a system that was largely pro bono publico into an intellectual pyramid scam. Academia has been slow to respond, mired in the web of prestigious citation. But maybe this is the end game.