Re: missing the point
Newer systems remain compatible with older ones. No backwards compatibility is lost here. This is about going forwards and advancing in time to newer system facilities. Old code will still run.
There are a few cases where backwards compatibility has been dropped. The historic gate A20 switch comes to mind. At first it was a motherboard feature, then a chipset feature before becoming a CPU feature when the memory controller moved on chip. At least it did until it was finally canned perhaps 10 years ago. Nobody really noticed, but how many people still work in 16 bit real mode?
Going the other way there comes a point where it makes sense to simply require new features. If you don't you end up with an ever increasing amount of code that is only used very rarely and in all likelihood not by the developers. Sometimes that code can be relatively self contained as is the case for FPU emulation for example, in which case yes it's a compiler switch. What about hardware AES? If you make the decision not to mandate it you have a decent chunk of code famously difficult to implement correctly that is lying unused for the majority of users. At a certain point it makes sense to no longer support options of interest to an ever dwindling number of users and make full use of advances in technology.