I have worn glasses or contacts for the majority of the time for about 18 years. I swear that my eye site without glasses has gotten worse. Though it may have with age I think part of it is that my eyes have gotten lazy since the correction.
Our eyes contain muscles that are used to focus. Like any muscles, they will atrophy if not used.
Not that my eyes will ever get to 20/20 naturally I am going to try and improve them. I have stopped wearing them unless I absolutely have to wear them.
It seems my vision has improved over the short time I have been doing this. If I really want to step it up, I can add eye exercises.