I think this is an excellent trade for Arizona.
Granted, Ertz (31) has declined the past two seasons, and dealt with injuries. However, Philadelphia's offense and QB-play have also declined over that period. With defenses focused on Hopkins, Green and Murray's scrambling, Ertz will not garner much attention; and figures to be a top weapon for the Cardinals.
Even in his least productive seasons, Ertz's numbers were significantly better than Maxx Williams.
Williams was playing well before his injury but I think AZ has up-graded at TE.