For sure this doesn't replace mechanical cooling.

But efficiency is extremely important in this context, not just for saving energy, but because the inefficiency manifests as heat generated, which undermines the intended refrigeration. So as far as Peltiers go, a doubling of efficiency is like a 3x ~ 4x improvement in effectiveness. Peltiers are already used for cooling in some contexts (eg. cooling CCDs) and this greatly grows the envelope for where they can be effectively employed.