You are missing something: the companies that build tunnel boring machines already have engineers who know how to make the machines looking into the problem of making their designs better. It isn't clear that there is a significant improvement available.
Rockets were a small enough niche that there were improvements possible if someone invested in them. It isn't clear if such improvements are even possible. (meaning they might be)
Right, but existing boring machine manufacturers don't have an infinite money pile to burn through, so they can't afford to try all the things that TBC will be allowed to try simply because Silicon Valley will afford them a bottomless pit of cash (until the next recession).
Rockets were a small enough niche that there were improvements possible if someone invested in them. It isn't clear if such improvements are even possible. (meaning they might be)