Isn't this an argument against any company/monopoly? No company forced anyone to buy anything.

This argument only works if there is sufficient competition. I would say that Apple has enough competition to agree with the argument that "you don't have to buy an iPhone".

How is Apple a “monopoly” with less than 15%?

Most people are “forced” to get internet through their cable company because there often isn’t a choice. How many people have to buy a phone that on average is twice as expensive as the alternative and then complain about it?

Mono means one btw

Neither did AT&T.

If you wanted phone service, you had to buy it from AT&T - because of a government granted monopoly.

Funnily enough, the government-granted monopoly only came as a consequence of a sweetheart deal that AT&T cut with the federal government- after the gov't first tried to pursue antitrust to halt AT&T's monopolistic activities:

https://en.wikipedia.org/wiki/Kingsbury_Commitment