Sure, there are lots of dead branches in the small arms R&D tree. A lot of the failures were reaching a bit too high. But there have also been lots of advancements.
The M24 got re-chambered in 300 WM, then they switched to the M2010. Sure, the MSR/Mk21 was very short lived because Remington sucks and it turned out they didn’t meet requirements. But now the Army (and SOCOM) have contracts with Barrett for the Mk22/PSR that will come with quick swappable barrels/bolts for 308 (for training), 300NM and 338NM.
The Mk22 is slated to replace both the M2010 and the M107 in SOCOM and big Army. The USMC is considering it too.
Now, the 338NM MG is SOCOM specific for now, but the Army is looking at it. Since the newly fielded 6.8x51 XM250 kind of takes some of the use cases from the M240, I could see big Army wanting a little 338 Norma action to replace both the M240 and M2. That said, there is the tricky bit with the M240C being used in tracks and that’s a whole big can of worms. They may just re-chamber those to 6.8x51.
As for 6.8x51, it very well may have a short life, but I don’t think so. It’s already being fielded. I do think there’s a chance the XM7 is short lived, but I think the caliber and XM250 are here to stay for the foreseeable future. The only qualifier for 6.8x51 being short lived is if Lake City can’t get up to speed on pumping them out.