[makomk]

Well, it should mostly see the real Bitcoin chain as valid until it doesn't. Older full nodes have a bug where they can't handle chain reorganisations properly with blocks that are the full 1MB in size, so the moment they see an orphaned block they break horribly. (Relatedly, if any big block pusher argues increasing the block size is just a simple matter of changing a few constants, that's a good sign they don't know what they're talking about.)

[MertsA]

Ah, good point I remember something about that bug a number of years back. Point being SegWit is designed to be compatible with old versions of Bitcoin. Bugs aside it's still a valid chain, old Bitcoin nodes just break on blocks that are considered valid by their own rules.