What's all this about instructors?
90-day rule is (was!), simple. If you didn't have three take offs and landfings as sole manipulator, then a few solo circuits and Bob's your uncle, in climbs passenger.
Yes, if you are hiring, or in a syndicate, local rules may require an instructor for a checkout. Or you might feel rusty and feel the need for an instructors services.
But instructors were not necessary and rule was simple.
Now, more ways to fly illegally (by accident through ignorance or misunderstanding) and so more risk that passengers will be uncovered by insurance.
I couldn't get a job with the CAA because I wouldn't be able to make this stuff up!