This trade has to do with the power structure of not only the NBA head office, but also that of the Lakers: it's now the Jim Bus Era, and it might be another sign of shifting focus to Bynum as THE big man, and franchise standard.
Is that a bluff? The organization has been selling it for long enough.
So far as Gasol "clearly" declining? His regular season was on-par with what he'd achieved before in forum blue&gold, and it was only the second season in his career where he averaged a double-double (not animal-style). He's played in under 800 regular season games, and much of his worth his footwork, size and overall skill-based rather than athletic.
He declined as the playoffs approached last season, and was miserable in them. But that was matched to the team's performance, which speaks both to his worth and the factor that likely truly killed the Lakers: they'd just come off 3 straight seasons where they made the Finals. Teams almost universally collapse the fourth year following such runs in the modern era.
The irony of people complaining about this trade being vetoed by the NBA, is that the Lakers never should have had Gasol to trade in the first place. Note that the media is more upset about this than they ever were about what went down with the league's pushing of Gasol to LA and Garnett to Boston.