Speaking for myself in case I was a new player, 10$ just to get going probably would be the absolute max.

Yes, this was how long ago? Times have changed now. You get so much value for your Spell-book. You can join a guild, the card designs improved, lore, land-worker and so on. Isn't a Spell-book increase of let's say from 10$ to 12$ justified? This game is sooo great now compared to what it was when I joined six years ago. The player who trusted and took the risk to pay the 10$ back then should be rewarded. The values of their Spell-book should increase a little bit.

However I don't think right now, bots buying 10$ spellbooks to try and milk rewards is a viable strategy

I don't know, but the Spell-book increase will end new botfarms one and for all times. It will become an uphill battle for them, so new ones will stop trying. New single bots specially created for Splinterlands can't be stopped by this measure. But we don't get new farms at least.