But that was kind of my point - Ryzen 5 3600 I can currently find for £180, while an i5 9400 is £198. If I now have to get a £30-40 dedicated GPU for the first one, Ryzen ends up being more expensive(not to mention that a super basic GPU like the GT710 does actually struggle driving a 4K screen, while the Intel 630 does not, but that's a minor point)

Not everyone is willing to buy old underpowered yet power hungry cards. You'd waste your time searching, you card would'not support 4k and VP9, no warranty. It gets especially expensive if you purchasing machines for some software developing shops - you want to have similar configuration on each machine, to save on IT support.

You're not wrong, the GT 710 only does 4k at <=30 Hz, which makes it a terrible choice for general purpose computing at this point. It also doesn't do VP9 acceleration. The GT 1030 does these things, but it's also twice as expensive.

What is "general purpose" computing at 30+Hhz?

710 plays 1080p YouTube and non-graphics intensive games fine.

I looked at this exact thing yesterday and those are currently the best option. They're also passively cooled which is nice. Having to use NVIDIA is annoying because of the story around Linux support. I mostly worry about these being old new stock and having poor reliability because of that but maybe it's fine.

New old stock is probably not a big deal for GPUs ... There's not a lot that's going to go bad on one that's just been stored for a couple years. From arcade collecting experience, big things that go bad in storage are batteries, and capacitors, and maybe dust accumulation. But a GPU doesn't have batteries, and capacitors usually take 10+ years to degrade in absence of heat from usage or bad chemicals (and heat makes that worse, so you'ld already know there was another bad cap scourge).

You can also avoid all that by getting a new design GT 710 https://www.anandtech.com/show/15713/asus-launches-an-old-gp...

Except for a couple of very new cards, most GT 710 cards don't have a DispayPort output, only HDMI. I also wonder if they support HDCP 2.2.