By all accounts the reason we still have to use a piece of equipment that was used by WW2 bomber crews is the old power issue. The CAA think that the whizz wheel is more reliable as there are no batteries to run out. Fair point, but only to a point. There are that many nav aids available these days that if the battery packs up then look at using another aid. Carry a spare set of batteries. Use VOR/NDB, call ATC, the options are endless really, and the whizz wheel isn't that accurate really. It needs to be supported by mental arithmatic as well. Not too hard on the ground, but a complete pain if being banged about at 2000 feet!
However, as we know the powers that be aren't keen on change, so the whizz wheel will stay for the forseeable future I imagine.