You're talking about Greatness, while the thread is discussing Power. As admirable as your critique is, they're two separate things.
Now if you had mentioned the Exile's ability to drain the Force from others, thusly adding it to his own - then that would've been a valid argument for his *eventual* power.
Pre-Mandalorian Wars, the exile was merely a "mediocre Jedi". Theoretically the Exile's Power could continue to increase indefinately....but we simply don't know his fate and hence, whether it does or if he 'becomes one with the Force'.
-----
I believe this was a mistake of George Lucas' part. A sign of a great story/event (especially one invoving prophecies!), is to "allow" the reader/viewer to acertain their own interpretations - such inexactness lends strength, as it add to the mystery of the Force and the mythos of the Jedi. Instead, 'Anakin as the Chosen One' is now press release info as George Lucas has publicly anointed Anakin thusly so.
*shrugs*
If George Lucas want to f*** with his creation, who are we to stop him?