Just decide what role you want a goblin or kobold, or group of goblins or kobolds, to play in your game, and that's their "alignment." Who cares what the MM says? It's your table.
The issue I've always had with default alignments in a stat block has little to do with the philosophy of 'alignment' or 'default alignment' and more to do with the fact... it's the only thing in a block of RAW text that isn't actually RAW and barely even qualifies as lore (except for extremely specific types of monster, like those that are basically "alignment, embodied" to a certain extent).
Most people only interface with the block. If the block has a default alignment on it, that's the assumption they'll make - it's one of the reasons I liked that dragons in Fizban's had "typically" beside the alignment. When somebody brought up those blocks, that reminder was always there. It's the kind of thing that needs to always be there for it to stick in the mind of a general audience, because the
lack of that verbiage is what sticks in their minds without it. A paragraph in the preface somewhere just doesn't cut it.
We can all shout at each other about how "it's just a suggestion" until we're blue in the face, but it's useless in a place like this because the people who'd need to hear "CE in the goblin stat block doesn't mean you should assume they're all CE" aren't the ones posting or reading here.
From what I've seen, we still have not progressed to a general awareness in the fandom/playerbase that default alignments aren't even really a lore recommendation anymore. Many players encountering monsters/NPCs that deviate from the default are likely to still just assume it's DM fiat. Again, a paragraph in a section only a DM is going to read just doesn't cut it.
However... for something like this, a change to goblins and kobolds, a new default alignment probably
should show up in the stat block like this, for that same reason, even if you dropped most default alignments from the blocks.
So I quite like these changes (goblins being CN fits well enough, and kobolds have genuinely felt to me like they were diluting what "Evil" meant as LE - they're either bumbling goofballs or traditionally depicted as defending their homes (sometimes both), even as enemies of the PCs). I'm actually more disappointed we lost "typically" with the dragons. If alignment needs to be in every block, IMO it should have "typically" next to it except for those aforementioned "alignment embodied" examples.