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I really do have 10base2 around my walls at home. How long before the 5 and 5e is obsolete too?

Not seen anything capable of connecting to 10Base2 (or 10Base5) for many years now.

Which is a shame as a few years back I wanted to be able to cheaply connect some farm buildings together and these with their single segment limits of 185m and 500m and (once) readily available network cards made them good candidates, albeit limited to 10Mbps...

As for the answer to your answer, I suspect that if you have 10Base2, you already know that 5 and 5e are unlikely to become obsolete anytime soon, particularly given 5e can support 1Gbps upto 100m. However, I anticipate we will increasingly see laptops following tablets and only supporting WiFi (unless you want to play around with USB dongles etc.). So overall we can expect the price of Ethernet over twisted pair to increase and necessary components become less available in the retail/consumer sector.