E* is starting to charge for the 501/708 DVRs that used to be free of DVR fees? I wouldn't be surprised to see them start charging a DVR fee for 211/211ks at some point.
The 501 is what, 10 years old now give or take? How long has Dish been charging a DVR fee for newer receivers 5+ years? And even with the plans to start assessing the fee, I think they are crediting it back for another 18 months or so to keep the no increase pledge.
Dish could well change the fee tomorrow, or the day after the rate guarantee expires, or stop offering 211 replacements if the unit dies and force a fee change that way, but in general they have a pretty good track record on grandfathering in old plans.
I wouldn't go in with blinders on thinking there would never be a change, but I wouldn't worry about it too much either.