You can change your advertising ID any time you want -- and almost none of their revenue comes from ad sales. In 2016, $190B of their $215B in sales was definitively not from ad sales, and the $25B services category that would include ad sales also includes the percentage of sales from the App Store, Apple Pay, Apple Care, iCloud.. etc. Ad sales are immaterial to their business model.
Immaterial would be a wrong word. Negligible might be better. Negligible right now would be perfect. Let's not be naive, they have a full advertisement department trying to monetize their platform and increase the revenue. They also tried to compete with Google for six years for the same, but failed in the in-app advertising. They are still trying to expand their advertising in other areas though:
And in any case, a company who makes 99.9% of their revenue off of non-ad sales is definitely would never be categorized as an advertising company. My grocery store is paid to put crest at eye-level, that doesn't make them an advertising company...
You're right, I wouldn't say your grocery store is an advertising company because they put Crest at eye level - I'd say they're an advertising company because they likely sell your loyalty card data to 3rd parties for $$$. Adtech is eating everyone's data.
IDFA is still going strong on Apple phones and is likely to continue into iOS 11.
