Keep in mind that the printers are getting regular upgrade cycles. A 2019 i3 MK3S is nothing like a 2015 i3. Firmware upgrades are released every couple of months. So continuous testing is still valuable.
Managing a printer farm sounds like a lot work, but they did do that work and can reap the benefit that prototyping and manufacturing are the exact same process. It's like saying "why does Google have all these servers when they could just buy them from AWS?" Well, they got good at building datacenters before you could just rent a datacenter from Amazon. If you're good at something, keep it up!