The modern version, a plea bargain, is motivated by the threat of a much more severe sentence if the defendant insists on a trial and is convicted. This is looking at it backwards. It's not a threat of a "more severe sentence", it's a threat of a default sentence (the same sentence the accused would get if plea bargaining didn't exist at all). The plea bargain is an offer of a lesser sentence. That distinction makes…
The "default" is updated when you learn about plea bargain... You are now comparing options relative to plea sentence. If the best option "walk free" is uncertain even if you're not guilty (what if the other guy has a better lawyer or something) then plea sentence is the best certain option.