I switched from Chrome to Firefox in 2019 because that's when Google adopted Manifest V3 and I never looked back. There were already articles then describing how it'd break ad blockers, and Firefox had at the time just recently released their "Quantum" overhaul which drastically improved responsiveness.
I'm a bit surprised it took five years for Google to drop support for Manifest V2, but the threat has long been there.
My best guess: whatever they're filing now was so exhaustively researched that it took months to prepare the strongest case they're able to make, possibly delayed by the lawyers working on several other cases. Plus waiting until sales have dried up can maximize damages.
Another possibility is that Nintendo/TPC is planning to make some big Pokémon announcements soon and wants to target this shortly before their own new games to reduce competition. Palworld might seem like more of a threat to the execs now that Pokémon is nearing a major release than it was in the middle of a long drought for the series.