Botnets exist today and mine today, this is not fiction. That said, if we assume AI will become more advanced like we do with CPUs becoming more efficient then yes eventually there is a tipping point. Bitcoin will not be mined by human beings in 2100 as there is no in my opinion it will take AI that long to figure it out. AI does not have to be AGI, but can be narrow insect level intelligent AI, and that alone would be enough for a botnet to take over mining. The thing about Bitcoin, it could evolve into a fully autonomous agent while POS or DPOS will always be semi-autonomous until the stakeholders are bots.