Fundamentally I agree with that shorting the 20 year is a great play over the next 2 years.
BUT... the TBT is a VERY flawed instrument. Have you considered the tracking error of the TBT especially over longer time periods? Check this 2 year chart of the TLT vs TBT. The TLT is at around -12% return over 2 years. You would expect the TBT to have a nice gain yet it is at DOUBLE THE LOSS of the TLT.
Shorting the TLT is a much better idea than buying the TBT over a long time horizon - this chart doesn't lie. Or even better buy TLT Puts for leverage.