This is an interesting topic, and I think the answer relies on the item in question.
Smart devices are complicated in this respect, because of the dependence on ongoing license agreements. It's perfectly possible for a manufacturer to end or lose an agreement with a provider and then for existing owners to lose that particular functionality. There is also the fact that, even where a licensing agreement remains in place for the brand, products beyond a certain age/operating system will no longer be supported in future developments. This is where the lines now blur between more widely adopted consumer products, such as mobile phones and laptops, and specialist hi-fi.
I haven't kept anything long enough to comment on long-term reliability, but I have had plenty of experience of older secondhand kit. My PL12D is older than I am and still going strong following a service before I purchased it. I don't envisage having any issues with it other than basic maintenance, but even if I do I can't see it being anything that can't be repaired.
I've had older amps of various makes and never experienced any problems as such, but I'm sure some of them would have benefitted from a service.
CD players are more hit and miss, but that applies to recent/new ones as well as older ones. I had a Rotel CD11 Tribute that used to stopped suddenly mid-CD without warning. It went back to Rotel via Richer Sounds and was successfully repaired (new loader described), but I have also come across others who have experienced the same issue (including my own Dad). The biggest long-term issue I've come across with CD players is an increase in noise and clunkiness, with only the odd one here and there suffering from read errors.
I've never come across problems with speakers unless they've been abused, either physically or from being under-/over- driven. Disintegrating foam surrounds are an issue on some older speakers I know, but I've never experienced that myself.
My expectations from my current system are that it should all last me a long time. The Denon has a 6-year warranty, and if it serves me for that long I will have got my monies worth out of it - £299.00 over 6 years being £49.83 per year. I expect the Missions to see me through for as long as I want them to, and I wouldn't be surprised if that's for 20+ years. I've owned enough speakers to know they're special enough to be a very permanent fixture. If I get 10 years with them they will have cost only £30 per annum.