So why did half the cast leave?

Not sure.....

I no longer watched after Judd Nelson left.

The show's cast seemed to be very close - and aware of a problem with David Strickland. I think the cast simply fell apart around his death; too many left and with so many irrational cast, story and setting changes rung in too quickly... "Suddenly Susan Wasn't".

"Vademecum..."

It was an attempt at retooling. There was the opinion that Susan and Jack lacked sufficient chemistry as the will they or won't they couple. The initial plan for Susan's love life during the show's final season was to have her involved in a triangle with Oliver and Luis, which necessitated the exit of Maddie, who many felt never caught on anyway. Ultimately, the Luis angle was dropped and Oliver became Susan's sole love interest.

I thought it was just a means to cut the budget. The ratings were falling and they were trying to prop it up.