or maybe the mini pcs started becoming available at all. I think these were a concept since core 2 duo and first gen i3 era. before then the chips were too much wattage for a mini pc. and the first generation mini pcs were also costly for enterprises to adopt in bulk. its only after 4th and 6th gen intel that these were adopted in large quantities in enterprises and once they dumped those in the used market they became comparable in pricing and somewhat in power to a rpi.
Used powerful PCs have always been available for dirt cheap on eBay even when the first pi was launched. just that mini pcs are a more recent thing.
As far as mini-pcs... The first Intel NUC was a Sandy bridge, i.e. Gen2, FWIW. And we did have the 'Mac Mini' as far back as 2005, buuuuut that wasn't cheap because Apple.
I think SSDs were another big part of mini-PCs though; it's easy to forget for granted the risks of having a hard drive in a poor thermal environment, as well as the general size constraints one imposes. [0]
[0] - Surrrre, you could use a 2.5 inch HDD instead, but 2.5 inch HDDs almost always were slower than a 3.5 inch HDD (excluding the VelociRaptor, but that would only be more demanding as far as thermal...)