Not obsolete. The mew books are 99% the same rules. Changes are minor and with two noteworthy exceptions (removal of alignment and spell schools) most Changes are to individual spells, feats and classes.
The old content is still perfectly viable to play, the new stuff is readily available for free on AoN just like the old stuff.
But it IS a new book so you'd have to buy it if you want it.
Yeah, the remaster is basically just a combination of "a large errata document" and "different collation of the same stuff."
Like things that were in the APG are now in Player Core 1, things that were in the CRB will be in the GM Core or Player Core 2 and not Player Core 1.
But the new stuff is 100% compatible with the old stuff. There's just a little bit of conversion work involved (like replacing alignment damage with spirit damage, since alignment doesn't exist anymore.)