Still selling brand new machines with 5400 rpm HDs. In 2019. It undermines the brand. Any Mac a customer buys should have good enough performance. 5400 spinning disk hasn’t been fast enough for a decade.
it's SATA so you can replace it with anything you like; if you're not fond of paying the extra $200 for 25% of the storage capacity. A bit like buying an OS-less PC. But I agree full-heartedly that it's counter-intuitive to the brand.
I’d just throw a thunderbolt or usb3 SSD in a port instead. I boot off a USB3 SSD on my Mac mini and that seems to work fine, despite not being quite as good as an internal or TB drive would be.