I think the lack of follow through problem is a myth.
INTPs are no less likely to follow through on a myriad of ideas than the other types on average.
When you have a lot to choose from, only a few things can actually be done. What people usually choose is what they are comfortable doing.
INTPs may be one of the more risk averse types, so what they follow through on, will tend be what they consider safe. This makes it seem like they are not following through on anything, but really what is happening is a choice of the safe and comfortable.
INTPs that I know, including myself, dream in vivid detail about humanity's future, and can express strong urges to take part in making that future become true. They may be more obsessed with this future than most people.
This may add to the image of someone who lacks follow through. People, in general, do not follow through on grand visions of the future of humanity. The difference may rather be the anount of time spent thinking and talking about these sorts of things.