I can speak only as someone who purchased his various asset packs here, and has seen his posts on Reddit:
You can download any updates to this asset pack. Note that once he hits 20k assets in this collection he's planning on starting a new one, but even when I first purchased the collection back in March it was a great deal, and it's only gotten better.
So yes, free updates to this asset pack, and you can expect at least another 2,000 assets to be added. Kenney is awesome, just buy it. :)