As far as extensibility and/or openness of the platform goes, that may define general purpose computing for you but the industry has abundantly shown that it doesn't for most people.
I don't think folks are doing programming and graphic design on their phones...
Right now it takes about 4 years to sell one billion PCs[1]. Considering that includes business sales, a significant chunk of that includes people with multiple PCs.
Compare that to smartphones, which sell well over a billion per year[2].
[1]: https://www.statista.com/statistics/272595/global-shipments-...
[2]: https://www.investopedia.com/news/smartphone-sales-be-flat-y...
Now you are using sales as a proxy for users which is nearly as false.
People buy and rapidly dispose of phones for a variety of reasons.
They break, are lost, get stolen. Having a new model is a status symbol. Current models are supported by the manufacturer for only a few years on the Android side. Newer models are significantly better than last years.
PCs easily last 5-7 years. Decreasing revenue from new models annually doesn't imply that the users or the platform is no longer relevant.
Reports of the death of the pc are greatly exaggerated.
Obviously a day may come where people plug their phones into docking stations that are equipped with a keyboard and a monitor, but it seems likely that we'll stick to lightweight laptops for a long time.
The people who learned to program in this way will appreciate the ability to control a device in this way and will naturally steer towards platform allowing them to do so.
Moreover, even Apple is changing these days. Making Swift available is a step towards more ubiquitous app development. And with Swift Playgrounds you can learn to actually program on your device, even though it's not the same as controlling it the way we're used to. It's not unthinkable they make further concessions and at some point iOS devices could be programmable to a certain extent.