If you look at the Intel transition, in 2006 they transitioned to selling 100% Intel hardware, in August 2009 you had Snow Leopard which was the first Intel-only OS but didn't introduce any new features you were missing out by skipping it.
Then in 2011 you had Lion, which was the first heavy overhaul of the OS and you needed Intel for it.
So they dumped the PPC users after 5 years effectively.
Now things are different, Apple is a much bigger company, PPCs were a very small market niche and now there are many more Intel Macs being used around the world, they can't dump them that easily.
Even more importantly, Intel Mac Mini and Mac Pro machines are still being sold, sure they're low volume products but somebody is paying very big amounts of money for them, possibly on leasing contracts, and they expect to have support for their workflows, and to receive at least security updates for 3-4 years starting from the end of sales.
So I'd expect smaller machines (iMacs, Macbooks) to be phased out gradually, Ventura would be the last OS for 2017 machines, next one would be the last for 2018 machines, and so on, then around 2026 we'll have the first macOS for Silicon only, and optional Rosetta (like Snow Leopard was), and around 2028 Apple could drop x86 binaries support altogether. Maybe 2030 for end of security updates on the last intel OS.