Here's one reason to think time travel a la Back to the Future or one of the truly great but definitely most NSFW episodes of South Park will never work: we don't have contemporary or historical reports of time-traveling arbitrageurs. Today's nominal wages are much higher than nominal wages in the 1970s, and obviously nominal prices in the 1970s are much lower than nominal prices today. I don't see any reason to think nominal wages won't be much higher in fifty or a hundred or two hundred or five hundred years. Rational arbitrageurs should be earning money in the future and spending it in the past. Since we don't have any recorded instances of that, I'm skeptical that we'll ever actually master time travel the way it's depicted in Back to the Future.
Of course, time travel might be so expensive that people might not be able to say "hey, let's go to the Sunbury Road Taco Bell in 1995 for lunch" (I was a regular frequent visitor in high school), but I suspect there are a lot of other markets where time travel arbitrage would be profitable. Long vacations and sabbaticals might be one example. If I just wanted to sit on a beach and read for six months, it would surely be cheaper to do it in 1955 than 2015. Would you also imagine a number of future music buffs who would be willing to pay enormous amounts of money to see Elvis live? How many hipsters would pay enormous amounts of money to get an apartment in Hamburg in the early 1960s so they could say "yeah, I was a Beatles fan before they were cool"?
Using 2015 dollars in 1955 might be a bit of a problem as the government has since closed the gold window and as the printing is probably a little bit different, but this is easily solved with currency exchanges where I could exchange my 2015 dollars for 1955 dollars. The fact that we don't see the law of one price ruling all markets across all of time suggests to me too that time travel the way a lot of us envision it won't ever happen.
Of course, I could also be getting the time travel rules wrong. What am I missing?