ST's are not a good mechanic IMO to model multiple levels of success. The ST + SR gives you three levels: completely ignore (SR), and two different possible effects for the ST based on success or not. By removing SR, and using it to modify the ST, you artificially increase/decrease the success on the other two options in order to introduce a third. This is especially true if you try to introduce some modified version of evasion (to work on non-Reflex saves). It complicates the saving throw mechanic, as well, and for a game that is supposed to be simplified, particularly towards the player, that's not good. It will not make much of a difference for the average reader of a Rules forum, but I know there are many players (and even DM's out there) who think that even D&D's simplified ruleset is too complicated.
So, let's take a crack at it. Consider slay living. It allows a Fort save where success means you take some damage and failure means you die. It also allows SR where success means you completely ignore the spell. For simplicity, let's ignore all other things like AC, crits, death wards, etc. In order to use the ST mechanic only (multiple people here have suggested it), you would need to create a multi-level ST. As you decide on your rules to handle that, I'm sure you'll quickly see how poorly it models the existing mechanic, meaning that the existing mechanic is a good one.