If people can disable iMessages, and also use other messaging apps, what is the issue here? If people don’t want to be in the “walled garden” are they not allow

If people can disable iMessages, and also use other messaging apps, what is the issue here? If people don’t want to be in the “walled garden” are they not allowed to switch from iPhone to Android? Certainly someone willing to dish out the cost of an iPhone can afford an alternative device if they wanted one.