That's what happens when you quote only part of a statement. Taken in context, it was referring to a very real decline in expansion cards. Now that NICs (for WiFi) and SSDs have been moved into their own compact specialized slots, and Ethernet and audio have been standard integrated onto the motherboard itself for decades, the regular PCIe slots are vestigial. They simply are not widely used anymore for expanding a PC with a variety of peripherals (that era was already mostly over by the transition from 32-bit PCIe to PCIe).
Across all desktop PCs, the most common number of slots filled is one (a single GPU), and the average is surely less than one (systems using zero slots and relying on integrated graphics must greatly outnumber systems using more than one slot).
Even GPUs themselves are a horrible argument in favor of PCIe slots. The form factor is wildly unsuitable for a high-power compute accelerator, because it's ultimately derived from a 1980s form factor that prioritized total PCB area above all else, and made zero provisions for cards needing a heatsink and fan(s).
