The question of Scott Caan leaving Hawaii Five-0 is moot: the series ended its 10-season run on April 3, 2020. There has been no revival, reboot, or spin-off announced since.
Caan portrayed Detective Danny "Danno" Williams throughout all 10 seasons (2010-2020), appearing in 216 of 240 episodes. His reduced episode count in later seasons (contractual arrangement for family time in Los Angeles) fueled occasional departure speculation, but he remained a series regular through the finale.
Since 2020, Caan has focused on:
All 10 seasons stream on Paramount+. The complete series holds up as one of CBS's most successful modern reboots—240 episodes of procedural action with genuine character development and Hawaiian cultural integration.
Scott Caan didn't leave Hawaii Five-0—the show ended on its own terms with him as a core cast member throughout. Any future "leaving" scenario would require a revival that doesn't currently exist.