If Apple dropped the Mac, how many of those former Mac users would also drop the iPhone? (I would.)
And of course, Mac is the platform for writing iPhone apps. Indeed, Mac is the platform that Apple engineers use to write the iPhone operating system, which was based on and shares a lot of code with the Mac operating system.
The Mac is the lynchpin that holds the entire ecosystem together. It also provides a lot cachet to Apple. Most of the wealthiest and/or influential Apple users are Mac users as well as iPhone users. If Apple can't make a desktop computer anymore, the company's founding product, that would be embarrassing to say the least.
I would drop every device I have currently (myself and my family) which is too many to admit publicly
macOS is a major part of keeping iPhone users on iPhones, and in turn keeping Apple users buying subscriptions.
When you look at the most profitable demographics in the market (e.g. “computers over $1000”) Apple has a huge percentage of that market in both revenue and marketshare.