Here are a couple of data points from my experience.
I was diagnosed in 2014. A year before that my neurologist had suggested I consider getting a power wheelchair. I could still walk, but not well and not far. I had already had several significant falls resulting in broken bones, sprains, and bruises. Now, over 7 years later, I can still walk short distances, but must use the wheelchair for any meaningful distance.
When I first visited NuMotion (the wheelchair Durable Medical Equipment [DME] provider I used), the person I met with hastily told me that they could not get a wheelchair for someone who was still able to walk (I had walked in with the assistance of a cane). I told her that my neurologist had referred me. She had just wrapped up an extremely busy morning and had not had a chance to review my chart before I arrived. She quickly opened my chart, when a bit pale, and said "oh". She knew exactly what the diagnosis and prognosis were. She actually turned out to be spectacular and we worked together for many years until she moved to another company.
She worked through the process of getting me approved for a standing Permobil C500 wheelchair (equivalent to the current Permobil F5 VS model). This was through United Healthcare.
After 5 years, it was time for a new wheelchair. I worked with her again to get approved for the Permobil F5 VS wheelchair. This was through Medicare. The initial approval happened pretty quickly (a couple of months), but then we hit a snag. Medicare would only approve an F5 because I had a medical need for the standing feature. However, Medicare would not cover the cost of the standing feature. NuMotion was unable to tell me what my out of pocket costs would be until after I bought the wheelchair. They estimated it would be $18,000 to $20,000. If I did not get the standing feature (and pay that outrageous out of pocket cost), then Medicare would only approve me for an F3. I found the F3 unacceptable for various reasons. It took 9 months to get to this point.
So, in my experience, Medicare will approve you for a Permobil F3 (or equivalent) wheelchair while you can still walk. They will even approve you for an F5 standing wheelchair while you can still walk, but will require you to pay the cost of the standing feature. This may have changed over the last 3 years, though.
I think the outcome can be highly dependent on the person you work with at the DME. I was working with an all star and I think that really made a positive difference.
You really do want to get a wheelchair while you are still able to walk. Fine tuning the seating adjustments is critically important and requires repeatedly getting into and out of the wheelchair. Doing this while you are still mobil enough to transfer in and out of the wheelchair makes things much, much easier.
More importantly, I think a very significant factor in managing progression is to NOT overdo things. When I got my first wheelchair, I could walk for hundreds of hards if I stopped and rested along the way. But, it absolutely destroyed me. It is my perception that when I got my first wheelchair my rate of progression slowed down. I am not saying that will be the case for anyone else, but it sure was for me.
The power wheelchair has also greatly improved my quality of life. Instead of sitting at home or perhaps walking a hundred yards for some activity, the wheelchair allowed me to go for miles. I could shop again. I could hike again. I could go on my favorite bike trails again. The wheelchair allowed me to access our local ski area so that I could use an adaptive ski (biski) to go skiing again. I cannot tell you how liberating it was to have a wheelchair. Even today, I marvel at the sense of freedom I have once I get in my wheelchair.
All of that is said by someone who almost cancelled the first appointment for being evaluated for a wheelchair because the whole notion of having a wheelchair was so repugnant to me. I encourage everyone to get one early.
Steve