Actually the M240 has been in use with the US Army for quite a long time. The 240 was used as the COAX machine gun on the M728 CEV which came out in 1965. And the 249 was never a replacement for the M60, at least not in Combat Engineer units. Even the maintenance section at battalion level had 2 M60's and 2-4 M249's assigned to them during the early 90's. That was what we had when I was assigned to the 40th Eng CBT Bn, 1st AD from Jan 92 to Dec 94.
And from a logistics stand point, it makes sense to go to just one type of light/medium general purpose machine gun. The only advantage to the M249 is it shares ammo with the M16/M4. But I have never saw a 249 that actually fed reliably from M16 magazines.