First of all amortization is used in accounting because it is supposed to reflect how an asset loses it value over (and to distribute the cost of the asset over the time it is used) . A five year contract is not worth the same as a four year contract. That's just common sense. Depreciation is used on the balance sheet to reflect that. So yes, amortization is an accounting concept but it is supposed to reflect real changes in value as well. You can argue that value can increase over time (what if the player gets better? Not in Diego's case though), and there are methods in accounting that deal with that but that is beside the point.
I think most of your Diego points are valid but they can be turned upside down. Wolfsburg won the Bundesliga in 2008/2009 and have gone downhill since then, true. They got rid of good players but if Diego is a great player he could have done a little something to undo the effect that those departures had on the team. He didn't.
Diego will be playing for his 4th team in four years now. Like you said, it could be said that he hasn't had time to adjust to one team and therefore he doesn't perform. It could also be asked, why is he playing for his fourth team in four years? Why is he always being sold? Why does his value decrease every time? It's much more likely that he isn't such a good player than every team/coach being wrong about him.
But yes, Diego wasn't a bad player for us. But even being one of the better performers in a very very bad Juventus team doesn't make him a gem either.
I know what amortization is supposed to reflect, ref, and I know that it's relative to the length of a player's contract. Jeez. My point was that you can't use it as a blanket tool, because far more often than not it is an inaccurate indication of a players market value. As in selling Zidane, or buying Nedved - the examples are extreme, but you know what I'm saying.
Value is largely in the eye of the beholder, and amortization is just one tool used in a complicated process. Not to be a dick, (sincerely) but are you an economics major, or an accountant?

Because it's important to look beyond simple formulas, and baseline numerical statistics in these types of scenarios.
As for that line about players values increasing, etc., I'm not sure that is besides the point in this discussion - relative to the price we sold Diego for.
Even if you use amortization to justify the Diego price, and adjust the figures, we still would have been short roughly 5M. We should have given the player a second year - look how much Melo improved in year 2 with us.
Diego's value may have gone up (from the 15M figure); I doubt it would have declined as he acclimated to his surroundings.
And just as a relatively
pointless aside...
Considering all of the issues we had midseason after Q went down, etc., I have a feeling that keeping Diego around may have been enough to get us into Europa. Seriously. I think he could have been the difference for a couple more wins. This is all silly and hypothetical, but imagine we had kept Diego and not purchased Martinez? People can argue all they
want about Europa being a shit competition, but playing in and potentially winning a European tournament is good for team building, morale, recruiting players, it's a modest financial boost etc. Continuity is so crucial.
So how do you account for those values? You can't really, because it's all silly hypothetical nonsense at this point, and we don't know how it would have affected the team, and subsequent transactions, etc.
But it still makes me wonder 'what if?'
As for Diego being passed around, I don't like basic stats like '4 teams in 4 years,' when I know some specifics about the actual situation. As in, both of his previous teams were in a total state of disarray, bordering on incompetence. The 4 in 4, that's something I would expect someone who hadn't watched the player, or didn't know the other teams' situations to say. I know you know the player and watch Juventus, so for our purposes, I think that's a bit of a cop out; it makes Diego appear much worse than he is. The only two teams who have been displeased with the guy, were in the process of basically being run into the ground while he was there.
Anyway, I gotta do a bunch of work. Have a good afternoon, man. Sorry for the lengthy post, people.