If this was common, password hackers would encode such algorithms into password hackers, then once your password is leaked from one site,it's game over (I suspect the set of human brain easily executable algorithms isn't that large).

That’s great but for 95% of the population they’ll never do that. They’ll come up with a generic low security password that’s easy to both remember and type. And they’ll reuse essentially that same password on every site.

So, for your practices maybe a password manager is a step down. But for 95% of the popular it’s a step up.

Security is a spectrum, and password managers move most people to a more secure stance. You can yell at people that they can be more secure until you’re blue in the face, but most of them will shrug and ignore you.

I see, you left this out of your original post and it would have had me reply entirely different. This certainly can work for you and makes sense, expecting everyone to do this instead of using a tool to do this is unrealistic IMO. Everyone thinks about things differently and many people "dont care" about security and just want a password that works.