I think the important of defining peak is that currently arguments are way to influenced by individual matches and match ups. If it is stated in the rules that peak is 3 years, then the person would have to bring up evidence of a way longer and realistic time perspective.
It would allow for people to research these 3 years in depth and portray it and it would be very helpful when you have an underrated player at your hand. "He did have a 3 year peak even if the misconception said he had a short peak" etc.
Pippa is such a great example of it. He chose Adriano and was completely slated for it even if he brought forth great arguments for his case.
Which could be interesting and enlightening and all sorts of nice - but it could also be boring as feck, having to sift through (no doubt handbaggy) posts in which the managers attempt to "prove" their man was in fact consistently grand for three years.
If a player's consistency at a high/peak level is in question to begin with, it's going to be very hard to prove beyond a reasonable doubt that he WAS, in fact, consistently very good/great - it's in the nature of the thing itself. You can always bring up this or that to discredit this or that - see the Adriano thing. The bottom line will remain that Adriano's status is tainted because he declined dramatically - and nothing is going to change that. Having someone insisting that he's wrongly labeled will serve no purpose beyond pissing people off. In my opinion nothing very good or interesting will come from such an exercise in futility.
EDIT
Just as an example to illustrate the difficulty in "proving" that someone was consistently better than he's given credit for (from a pro player perspective): The debates over Nani's actual level have raged on here for years. His detractors maintain that he's always been on and off, that...something (lack of football brains, deficient mentality, innate propensity for blowing hot and cold, etc.) has always held him back, preventing him from becoming a truly top player. His apologists, on the other hand, will point to stats: For the seasons X to Y he grabbed more assists than A or B, had more "key passes" than C or D, played in so-and-so many crucial matches, getting the man of the match award in so-and-so many of them...and so forth.
Nobody is ever going to let go of that former impression (of a generally inconsistent player) once it's formed - and no amount of statistics will ever change it. It all depends on what the expectations are, where the bar is set, etc. You can't prove or disprove that someone is or is not at the level of X - which is what we're talking about here in practice: It will always come down to a comparison. Who fields the strongest team? Who has the best pair/trio/quartet of attackers? Is NN a "top, top player" or is he merely a "top" one? Forget it. Look at the Stevie G debates.
In short, it makes no sense to introduce hard criteria in an area which will always remain soft by its very nature. That, at least, is my opinion.