There's also the cost and amortization thereof. Admittedly mine are different than most here. For my dual GNS upgrade 14 years ago, installation and all, my cost was $3,600. I've been using it to one degree or another for 14 years, for a yearly amortization of $257/year.
For those of us with the original six-packs, which includes most GA planes flying today, any addition/change for a newer navigator and glass panel is going to have WAAS. I see these planes showing up on ramps and in shops all the time. So some 1978 C-182 or Piper whatever is going to be better-equipped than perhaps a much newer DA40.