Knee range of motion has clearly been recognized as a very important part of function. Without it, walking, stair-climbing, and everyday activities can be difficult. Being unable to complete these tasks alone or even with some help can be very disabling.
For most patients, getting enough motion back to accomplish the type of tasks you mention is possible. It may take persistence on your part. A daily exercise program of range of motion and strength training is definitely needed. You'll need at least 90 degree of knee flexion to do stairs and another three to five degrees to rise from a chair.
For some people, deformity or misalignment at the knee may be holding back restoration of greater motion. A flexion contracture (unable to straighten fully) causes a loss of full extension needed for normal gait (walking). And without full flexion, going up and down stairs can be very difficult.
It's probably about time for you to check back with your surgeon for a follow-up visit. Ask about your limitations. You may need some additional surgery to correct a problem. Or perhaps another round of rehab is appropriate. The benefit you will get from obtaining additional motion is well worth the time and effort.