The typical stairway lift in which the person is seated is at best only a temporary solution. Transfers to a wheelchair are necessary on both floors, and as the torso weakens the PALS will not be able to be stable sitting in the lift chair. I installed one of these and Annie was able to use it for a bit over a year while she was still able to be somewhat ambulatory with a rollator walker. She used a small power wheelchair downstairs, and the rollator upstairs to get to the bedroom. Eventually she had to graduate to a rehab wheelchair and she was no longer able to use the rollator or stairway lift.
If your house layout has sufficient space at the head and foot of the stairs, then a wheelchair stairway lift will work long term. Adding an elevator is a good permanent solution depending upon budget and house layout. By way of future planning, my brother had one installed during a major house renovation, and home elevators are becoming more common as the baby boomers are forced to face the reality that we actually are going to get old and decrepit.
As others have commented, making necessary accommodations all on the lower floor is usually the best solution, but particular house layout and budget play a huge role in the best approach for each individual situation. If the house can accommodate an elevator, then that has the advantage of giving one access to the whole house, though eventually that becomes somewhat of a moot point.