"I'm projecting a $2500 saving in taxes ( she isn't wealthy other than her residence)"

You indicate the medical deductibility threshold for itemization means there is a significant savings, but you also state she isn't wealthy? Not that wealth has anything to do with whether this is medical or basis, of course.

"What millennial couple would want a house with an electric staircase?"

One with a multi-generational family, one with a disabled adult in the home, one with an injured person in the home, one with a pregnant person in the home, one with a disabled vet in the home. I can think of a lot of reasons this might be attractive. It depends on if this is just a fold-down seat, a full unit that impedes on the regular use of that same staircase, a secondary staircase, is it easily removable?

Yes, they should have a real estate agent give them info on this type of addition in their area. It doesn't have to be a full blown appraisal.