Or they could rotate where the hordes occur. Meaning that you have a short time to hunt each specific location.
Obviously they should only do this for relatively common shinies, but would give you multiple locations where you can hunt hordes without really devaluing the more common shinies that would appear.
For example they could remove Grow/Pix/spheal/sableye hordes and replace them with others. Thus never really destroying the value of any pokemon.