Maybe something like for every 10% of health you lose, you gain .1 more shields "upgrade" making 1 hp units basically 3.9 shields (like zealots) last a bit longer making them more cost effective. "What about immortals. They would be even more op." Emp, and fungal + zerglings. Everyone has a counter to everything. Protoss obviously has Void rays which are both air (attacking counter) and do bonus damage vs armored which immortals are.
Can't really think of anything else as a counter argument. Don't really see how this would "break protoss". In all honesty it would be fair seeing how literally half our health is relied on shields for both units and production.