While PS4 Slim is still doing the business, people underestimate the importance of PS4 Pro.
Baseline was 20% during it's launch window, but almost 18 months on, with all the 4K talk and 1st party software, that can only have increased. It's appealing to both new console owners and those upgrading from regular PS4. Upgrading within your ecosystem is far more affordable than switching entirely, with expensive hardware, and new software.
That's also one of the reasons Switch is a more obvious addition for PS4 owners. That system offers exclusive software and a unique form factor. Sure, you still have to part with a decent sum of cash, but you're investing in a genuinely new experience.