> Have you never heard the phrase "There ain't no such thing as a free lunch"?
Yes, it's a meaningless cliche which is often untrue (I've had many free lunches myself). But "free/gratis" does have a definition to mean "without cost or payment", e.g. how much it costs to acquire/download every version of iOS going back a decade.
Have you ever heard of alternative business models? Try Googling Android, Tizen OS, Web OS, Firefox OS for alternative mobile OS's, it's nearly impossible to for a mobile OS to become mainstream if you had to pay for it.
> If the OS is free then why not let it run on very old models, it will run very slow but that should not matter since the OS is free, right?
Because Apple has to support the devices it says it supports & they don't run acceptably. This isn't new, a lot of commercial software wont support software that doesn't meet a minimum hardware requirement.