So what exactly is patentable? There has been a lot of prior art on the topic.

You can create a patent that builds on other patents. You can make up all kinds of "what if" "inventions" and spend a lot of money acquiring patents for them, if you have the money to do this originally. Then it is a game of having the financing to "protect" those patents from infringers. If a corporation has more legal reserves than a patent holder, it can be a long drawn out legal battle where the corporation eventually wins, or the patent holder can just not fight and it ends there. This is the patent system.