A patent can stifle work without being litigated. See the recent kerfuffle with react and graphQL. The simple matter of a patent that might exist was enough for people to change tooling.
Patents for AI might very well cool business R&D into AI.
It doesn't work that way in this case. No ML researcher goes out of their way to look for ML patents because the penalty for knowingly infringing is much worse than the penalty for accidentally infringing. If nobody knows these patents exist, and nobody is litigating them, then they can't be "dragging down" research.
Patents for AI might very well cool business R&D into AI.