Metallica is decent enough. That's it really, they were a solid band that made some ill-advised changes before going back to a (mostly) solid band again, but in this day and age they are not especially exciting or noteworthy. From a historical standpoint, yes, but bands don't get to just coast by on their historical significance with anyone but their most slavishly devoted fans.
Of course, these "Metallica purists" the OP is complaining about tend to arise in discussions of contemporary and (this part is key here) mainstream metal bands, and the significance is thus: Mainstream metal sucks. Seriously, if people who don't listen to Metal have heard of it, the odds of a modern metal band not sucking immensely are practically astronomical.
When weighed against the best of the much less well known modern bands, Metallica and their ilk are just "okay"; compared to the crap that gets popular attention today, old school Metal kicks copious amounts of ass. The problem isn't with being a modern Metal band, it's that the modern metal bands everyone has heard about play shitty music.