If the Stryker won't resist .50 cal AP, there's little to recommend it over a stretched version of the Hummer with a TOW launcher on top. At least we can transport lots of those by air, and they have good cross-country capabilities
The Stryker should have superior cross-country to HMMWVs--although as you say, the HMMWV is pretty good.
There's a lot more bad guys running around with 7.62 & 5.56, etc than with .50/.51 cal and the Stryker stops that stuff cold--as well as shell fragments--so on balance the Stryker is better deal than a HMMWV for the guys that have to ride in something.