Ignoring the hider because TL;DR and random spoilers* [i]and[/i] you somehow read the wiki page on Mystery and got completely the wrong idea...

If Rhon was literally not falling apart, it would indeed get stronger (or rather, you could actually use it; age = power and it's already plenty strong and can overwhelm modern comparisons). But it's falling apart. Also mostly significant for its metaphysical purpose.

Actually extant Noble Phantasms that haven't degraded in power one bit (just their use being limited): Fragarach, Avalon, and Jack's knives. They all work fine, they're just not something people can use freely. 

*AKA: I'm not entering into a debate where other people can be spoiled.