Everyone's mileage varies but that's not uncommon.
I have a phone with a great battery, still strong, from 2011. I have another that's highly diminished.
The second one I ran into shutdown all of the time, the first, I didn't.
If you're usage is high enough to have to swap batteries in the middle of the day, and you wait until one is depleted before swapping, then that's killing the battery life.
As they go into deep depletion, their little metal tubes tend to deform. That will always accelerate battery death.
No personal skepticism required.
PS - I haven't seen 50 in quite a while, and I know a lot of people as well.