My theory is that time travel is impossible for one reason: If time is indeed just a measurement of movement, then there is no "there" to go to. You're already there; the furniture has just been rearranged. A few continents have drifted apart, a few mountains have arisen, people and animals have come and gone.
Even if one could inconvenience everyone currently alive long enough to un-re-arrange everything back to where it was 10,000,000 years ago, the people currently alive would have to go somewhere and wait, since it's the same earth as 10,000,000 years ago. Unless time travel involves creation of multiple parallel universes, so that the non-time travellers could continue their lives undisturbed while the time travellers pursued their goals.
And I'm only talkng past travel. Don't start me on the future.
I see a huge commercial application not so much in time travel, but in time stopping, which might also be easier. Imagine the volume of adult travel in search of a perpetual orgasm. It would easily surpass the Internet pornography.