Jim Fitzgerald wrote: There's a lot of flying to be done between now and 2020.
 Precisely!  And possibly well beyond 2020.   
No promises, and I wouldn't bank on it, but the history of these expensive aeronautical equipment mandates is that they get delayed before their original implementation date is reached.  A mandate that scrapped many aircraft radios because of frequency stability was delayed from 1990 to 1997!  If memory serves, the ELT mandate was also delayed.  
Will the ADS-B mandate also be delayed?  Your guess is as good as mine, but already Congress is starting to hear complaints:  See  http://tinyurl.com/mfemudu