You get the said 10 year old chip cheaper, with lower power consumption and smaller physical size, on newer and improved fabrication methods.
You don't, pretty much. Exceptions exist, but not when it comes to mixed-signal processes.
Raspberry Pi would beg to differ.
Plenty of lower power or older stuff (RPI included) use older nodes just because that's available. Microcontrollers tend to use higher nodes (22, 40, or 55nm) just because they don't need the super high speed stuff.
Also, the RPI5 uses 16nm, not 22nm. Still not modern, but not unheard of for stuff like SBCs where performance is not particularly important compared to cost.
Now that the law is close to coming to an end, the economics changes. Latest tech provides negligible marginal benefits to the median consumer. So now it is possible to think of commodotizing a single process and making the factories much much cheaper.