Try this on for size.
It’s 2266. Captain Kirk and the Enterprise are zipping through space, and there’s an emergency distress call from a nearby colony, under attack. The Enterprise diverts from its routine patrol. Had the Enterprise stayed on its course for another day, Spock would have detected a derelict Earth ship from the 20th century.
The Botany Bay.
Kirk never stumbles across the Botany Bay. Never wakes Khan and his followers. Never banishes them to Ceti Alpha V.
In 2285 Kirk boards the Enterprise for a routine training mission. Chekov, first officer of the Reliant, surveys Ceti Alpha V and declares it fit for testing the Genesis Device. There’s no Khan on Ceti Alpha V to kidnap Chekov, steal the Genesis Device, attack the Enterprise and bring about Spock’s death.
No Genesis Incident. No collapse of the Mutara Nebula into the Genesis Planet. No sacrifice by Spock to restart the Enterprise‘s engines.
In 2286 Hikaru Sulu assumes command of the USS Excelsior.
Kirk never stole the Enterprise. He never needed to. Kirk never destroyed the Enterprise. Kirk was never demoted for his insubordination.
In 2287 Pavel Chekov assumes his own command, the USS Yorktown. He rescues the hostages on Nimbus III when a renegade Vulcan named Sybok takes over the tri-government embassy.
Kirk accepts a promotion to Commandant of Starfleet Academy.
The long-range effects of this timeline are profound. Kirk would be remembered more for his long service to Starfleet Academy (from 2284 onward into the 24th century) than for his two five-year missions. The “heroic age of captains” would be more likely to refer to Sulu and Chekov than to Kirk. Kirk would have molded several generations of Starfleet Officers, but he wouldn’t have been the one to be there at the front lines, time and again, putting out the Federation’s brushfires. That would have fallen to the officers he trained.
I fail to see who doesn’t benefit in this timeline.