The “newer MacBooks” are unpopular is just a meme in the tech bubble. While Apple will stop reporting unit sells next quarter, they just posted last quarters numbers. There is no indication that they are “unpopular”.
That is Sales Volume, but its 100M Active Devices, is way slower than what most predicted. Apple took more than 2 years to add 10M Active Devices, on a ~40+M Unit sold during that time, that means 30M are either replacement or, there are quite a high churn rate to Mac. It is possibly the only reason why Apple suddenly come up with iMac Pro, new MacBook Retina etc. It wasn't the sales that matters, it was user leaving its platform. By previous trend and growth at a nearly consistent 20M unit per year, Mac should have at least 120M Active Devices by now, if not more.
Why does Apple care about active users for the Mac and not sales volume?
Active users for iOS devices makes sense. Apple gets recurring revenue from iOS users. Hardly anyone uses the Mac App Store or buys iCloud storage just for the Mac and OS upgrades are free.
You will need a user base for Software Distribution. Will you design your software for a 10M or 100M Mac user. If you are changing to a Services product that only oriented around your products, the more user the better.
I wouldn't care about the overall number of users, just the number of users in the market that I'm targeting. The creative market didn't abandon Apple when the overall Mac market was in dire straights because the market they cared about still had a lot of Mac users.
From what I can see, the consumer market for software is basically dead. Is anyone making serious money on non game personal computer software besides Adobe and Microsoft?
The smaller Mac only software companies are going by the "thousand true fans" strategy. Stay small and get a small loyal customer base.