Well look at it this way... our military does not still use muskets instead of modern guns... we use modern guns... because we've advanced beyond the tech of muskets.
Technically speaking, there's not much difference between a musket and a "modern gun". The main distinguishing factor would be the presence or absence of rifling - but some of the most advanced modern guns installed on main battle tanks are unrifled, for a variety of reasons. Essentially, just big muskets. Also, the practice of packing the propelling explosive in a casing that's integrated to the bullet may have been a passing fad: some modern guns return to the idea of loading the gunpowder separately, so that the gun becomes caseless (much neater and faster that way) and can perhaps adjust the amount of propellant used. Field pieces certainly go for the adjustable propellant trick now, returning to long-bygone days and showing there's not much new under the sun. Should the situation really be different above the sun...?
Anyway, if it's "a bit silly" that the Feds are stuck with phaser tech for 600 years, what should we make of the fact that the Star Trek galaxy at large has apparently been stuck with similar tech for billions of years? I mean, TAS makes it clear the Slavers had it a full billion years ago (while failing to make the original Niven point that the Slavers left no heritage) - how come the thousands of succeeding cultures never advanced past that initial work? Can it be that a Trek culture never comes to meaningful contact with its immediate predecessor culture, and fails to "have the baton handed over"?
My favorite phaser? Varies from day to day, really. Today, I value the ST3 guns for carrying the TOS heritage in a plausible manner, although I also have a soft spot for the barely glimpsed guns aboard the Kelvin for the very same reason.
Timo Saloniemi