Hey - sudden thought I just had. (Partly inspired by ArcaneSpringboard pointing out we don't care about the size of the difference in the opposed rolls.)
What if we did care? What if this is the way out of the Save-or-Die and Save-or-Suck dilemma?
"In order to turn someone to stone, you have to beat their roll by X. If you don't, they are only slowed. If already slowed, you only have to beat their roll by Y."
This is sort of how M&M 2e does things (only there it's just a save, not an opposed roll) and it works pretty darn well. Typically a Save-or-Suck power has three tiers of effect; one when they fail by 0-4, one when they fail by 5-9, and true suckage when they fail by 10 or more. If they've got the first tier of effect already, the other two get bumped down by 5, and so on.
M&M 3e combined (almost) all such powers into a new Afflict framework that let you pick the three tiers to taste, and that also works well.
Use the 5e opposed roll the same way, and things could perhaps get quite interesting...