Nobody's in a hurry to replace the handfull of .50 cals. FN invented a really good 15mm (.60 cal) round and HMG but no takers.
There's talk of replacing the 7.62x51 NATO (.308 Win) with 300 Win Mag now that it's used only at support and long range sniping level.
Who knows?
I'm peronally in favour of replacing that silly varmint round (5.56 NATO/.223 Rem) with the .243 Win with proper rifles and breeding riflemen again instead of sprayers & prayers.