Compared to Gattuso in terms of ability, Davids is the superior player but unfortunately, Juve was only ever able to utilize his defensive (actually, more like athleticism) qualities, never his offensive capabilities. The pitbull would have been awesome if allowed a free role to do both but he's never gonna find that in Juve's system or the Serie A for that matter.
Pirlo is an interesting example. He IS indeed a fantasista. That was how he started in Brescia, languished for years in Inter before joining Milan. What he found in Milan though was the same story - the immediate prospect of dislodging Rui Costa and even Seedorf et al in the attacking scheme was always gonna be difficult.
I think Ancelotti proposed a compromise to him at that point - regular playing time (something he needs above all after wasting so much time in Inter) BUT in a new role as a CM. Pirlo, in his mid-20s, with his attacking instincts, technical style, physique and mindset already fixed, was never gonna cut it as an orthodox DM. However, he didn't have to unlearn and relearn completely - those qualities were adapted to re-define that role instead. Suit system to players, not the other way round. The system itself is a big factor. Milan has the technical cast in their squad to carry Pirlo. We don't, which is why Pirlo and even Maresca will most likely fail in our current setup. In contrast, even Gattuso looks like a world beater in Milan with all those technically-gifted players around him (Btw, I thoroughly enjoyed how the Deportivo midfield monkeyed Gattuso around and long for the day Juve will be able to do the same) IMHO, from the "team" standpoint, having the right complements are much more important than having intrinsic qualities to succeed in a given position. The same applies to Trez.
While it is true Milan might not be able to compare with us in terms of defensive coverage and recovery, they don't really have to if they don't lose possession and/or can keep them for hours. Another distinct advantage of their system is that it preserves the durability of players since they don't nearly have to expend as much energy as one like ours. That might explain why Maldini, Costacurta et al are still functional compared to Juve's veterans in Ferrara and especially Montero, who seems in accelerated decline. Coupled with the training regime, that could also explain our high turnover rate of players, drastic losses in form (Appiah and co.) and the ridiculous amount of injuries which plagues us no end.
Anyway, back to Pirlo, the position change was a huge challenge and after being trampled most notably by Nedved (2-1 win at the Delle Alpi last season), credit must be given to the player and those who persisted in the experiment. Given that Ancelotti was a master in the DM role himself, there are certain advantages to be gained when there's direct coaching from someone who's an expert in that area. By and large, it was a very good decision. No need to be overly envious though - Zambrotta's conversion to LB was a similar masterstroke by Lippi (on hindsight) The largest beneficiary of them all though is the Italian National Team.
To cap it all, I feel that having a clear gameplan and addressing problems as a whole (not simply direct 1-for-1 player changes) is the way to solve Juve's problems.