I have my Fitbit goal set at 7500 steps which, according to the article and other sources I've read, may be the "magic number". Although that is not originally why I had it set there. I used to have it set lower at fewer steps, than as I obtained that goal on a regular basis, I upped it. I went into holding pattern at 7500, because I figured that was doable and on some days, it seemed 10,000 steps was just not possible and therefore, would prove daunting and I would get discouraged. But if I hit 7500, I could say goal met and try to go even further without any 'added pressure'. Surprisingly, I usually can hit 10K.
I agree with @ManicProgressive , I believe maintaining some level of activity does help with mobility issues, but also a whole lot of other health issues not to mention weight. Especially during the past year we've had, I've been particularly cognizant of watching the FitBit and 'upping the ante'.