Analogue TV
" In the era of analog television, new signals piggybacked on existing signals."
It was a long time ago so I can't remember; was UK 405-line analogue forward compatible with 625-line, or did that require a new TV to be purchased?
I think we all accept that nothing will be infinitely forward compatible; hardware often doesn't have the grunt for even the next generation. I remember when Elonex introduced a PC which had the CPU on an ISA card so it could be upgraded. No need to guess how that worked out.
What annoys me most is when manufacturers won't make things backwards compatible, not because they can't but because they won't or don't care; apps which need the latest OS for no good reason seem to be the worst offenders.