Scrapers can send these challenges off to dedicated GPU farms or even FPGAs, which are an order of magnitude faster and more efficient.

Lets assume for the sake of argument, an AI scraper company actually attempted this. They don't, but lets assume it anyway.

The next Anubis release could include (for example), SHA256 instead of SHA1. This would be a simple, and basically transparent update for admins and end users. The AI company that invested into offloading the PoW to somewhere more efficient now has to spend significantly more resources changing their implementation than what it took for the devs and users of Anubis.

Yes, it technically remains a game of "cat and mouse", but heavily stacked against the cat. One step for Anubis is 2000 steps for a company reimplementing its client in more efficient hardware. Most of the Anubis changes can even be done without impacting the end users at all. That's a game AI companies aren't willing to play, because they've basically already lost. It doesn't really matter how "efficient" the implementation is, if it can be rendered unusable by a small Anubis update.