Depends on your use of the word 'epic'
If, by epic you mean characters over 20th level. Then almost every session with these characters ought to be 'epic'.
If by epic you means as in 'this movie was an Epic motion picture', then yeah, overkill can sneak in. But in takes a lot.
In my campaign, my PC's have gone through 3-4 Epic plotlines in the 2 years I've been running the campaign. Overkill has yet to sink in (there are a few other 'epic' plots the PC's have heard about, one involving a flight of dragons, but they haven't investigated).
Overkill manily come with existing settings where the publisher needs to put out products on a regular basis. The most obvious is the Forgotten Realms. Where, if you pay attention to every novel & sourcebook that comes out; all of Toril narrowly avoids COMPLETE DESTRUCTION by the FORCES OF EVIL, once a week (sort of like SG-1 or Star Trek).
In my games, it's running multiple games & trying to keep the sperate over-plots seperate.