https://www.looper.com/1914586/why-david-caruso-left-nypd-blue/

David Caruso was the first star of "NYPD Blue," leading Season 1 as Detective John Kelly. Here's why he was replaced by Jimmy Smits' Detective Bobby Simone.

David Caruso walked off the set (https://www.quora.com/Has-an-actor-actress-ever-walked-off-set-and-never-returned/answer/Regi-Davis-1) of NYPD Blue (https://www.reddit.com/r/nypdblue/comments/1do20ev/why_did_david_carusos_career_sink_after_nypd_blue/) after filming his last scene (https://whatculture.com/tv/10-actors-who-walked-off-huge-tv-shows-and-why-2?page=3) on the show (https://ew.com/article/1994/11/11/nypd-blue-copping-new-attitude/?srsltid=AfmBOoqjOg-ii_GP1dSpQyPy4yonlk7QIUPTA9ybXJJC79nAau9DqLS2).

According to a Natalie Stone article on PEOPLE.com, August 18, 2016

“Prior to filming the second season, Bochco alleges that, “Caruso (https://en.wikipedia.org/wiki/David_Caruso#1990s) wanted to be let out of his contract (https://www.looper.com/1914586/why-david-caruso-left-nypd-blue/).” After Bochco declined and threatened to sue, Caruso’s agent presented a list of demands (https://www.tellytalk.net/threads/nypd-blue.3976/post-96452) in a proposed restructuring of his deal, which initially included “One, $100,000 per episode. Two, Fridays off. Three, a 38-ft. trailer. Four, an office suite on the lot, replete with his own development executive, for whom we had to foot the bill to the tune of $1,000 a week. Five, two hotel suites in New York when the company went there on location, plus a dozen first-class plane tickets. And lastly, Caruso had to have additional security to shield him from his adoring public.”

Bochco declined, and the agent presented a revised list of requests, which included, “the last seven weeks of the season off, so that his window for doing feature films would be larger.”

The showrunner had finally “had it” and opted to let Caruso go from the series once he had filmed the first four episodes of the second season.

“When he had shot his last scene of the fourth episode, he turned without a word and left the set, the stage and the lot,” Bochco says of Caruso’s last day on set. “He didn’t say a single word of thanks or a goodbye to his cast mates – nothing.””

Dennis Franz 0 (https://www.reddit.com/r/nypdblue/comments/199tayn/comment/kipjru6/) David Caruso (https://www.reddit.com/r/nypdblue/comments/1do20ev/why_did_david_carusos_career_sink_after_nypd_blue/) so difficult that he would've considered leaving himself (https://www.chicagotribune.com/news/ct-xpm-1999-07-12-9907130281-story.html) if Caruso (https://www.reddit.com/r/nypdblue/comments/1hhgcfv/david_caruso/) hadn't, so presumably Sipowicz would've been written out at some point.