Well...that'd work, I guess. Idk. Hard to say without seeing/knowing how many fights you're having per rest, how difficult the encounters are, etc etc etc. My sense though is that it would keep warlocks exactly where they already are; starved for spell slots and pretty underpowered.

If you're looking to experiment in your own game (as opposed to selling another DM on something), I'd probably think a little bigger. Something like keeping their SR casting as it is, but also giving them LR 1/2 casting progression so they can get some utility and defense.