What’s always interesting to me about Metallica is their willingness to reinvent. I haven’t always agreed with their reinventions but it’s great that they keep changing. Kill Em All is really their only “pure” thrash album. With Lightning they started adding, oddly enough, a lot of prog rock elements, getting longer and more complex while still thrashing away. Then of course they ejected the prog and went pop/thrash for the black album. then they went Nu. Now they’re thrash again, but with some different layers. Prog/ thrash is my favorite era of them, but there’s always something there.
Contrast this with AC/DC and their "if it ain't broke, don't fix it" philosophy who have been putting out the same album since 1980.