Season 4 is hit-or-miss, but Mike's character wasn't so much a stick figure that you could throw in any other character, played by any other person, and expect it to work the same.
Locklear herself didn't wreck the show - she's a good actress. But Caitlin - who was added to provide conflict - just didn't feel right.
Without Mike, the show nosedived. The Charlie character, which I don't recall if he was a retread of Mike or attempting to be someone different, just didn't work.
After 9/11, the show would likely have seen a ratings plummet - "Spin City" was always jubilant and fun, and in the city it was placed in having a major attack against it... even I would have turned off.
Friends carried on as normal and that was also set in NY. At the time it wasn't criticised, nor did it lose viewers.