The recent software obsolescence was actually hardware.
Snow Leopard ran on 32-bit. Lion ran on hybrid 32/64-bit. Mountain lion and above runs on 64-bit only.
Welcome to Rivendell, Mr. Anderson.
But the clever engineers have ways of making one "need" to buy new boxes. Even if OS X stays 64-bit, you can bet that that as Intel pushes their design rules (they're claiming 10 nm chips will be ready in 2015), they'll be changing memory interfaces and probably have PCI-EIEIO interfaces. They only make money when people continue to buy boxes to replace their "obsolete" boxes.
I wouldn't be surprised if changes to internet communications comes in the next 4 years or so (though I haven't kept up with what's being worked on there). MS tried to lock things down with their "secure boot"; locking down internet connections in hardware seems like something that companies and ISPs might demand (e.g. in an attempt to make things more secure from NSA and Chinese snooping). Maybe mandatory Smart Card authentication for connections or something. Maybe not, but the point I'm trying to make is that I'd be very surprised if you could take any PC/Mac purchased today with Win8.1 or Ubuntu 13.04 or OSX 10.8 and be able to keep it updated and "usable" with a current OS and current software in September 2021 by only adding RAM. Something about the hardware will make it impractical to stay current. RAM for it will be obsolete and extremely expensive, or OS updates won't be available, or ...
(We've got a bunch of Win PCs at work that we had to take off the network because we couldn't keep the OS and software updated and the weren't able to run later versions. The hardware still worked fine.)