I think a 4-5 year window makes sense for apple, but consider all the work in the background to get both platforms to work? it’s a daunting task to think about the work that would need to be done the further the M series processors pull ahead. The software inevitably suffers as considerations would have to be made on chips that just aren’t as fast/efficient.
4-5 seems long on the hardware side of things. Maybe if you're counting the time until 90% of the apps that are going to convert to either Universal Binary 2 or Apple Silicon only binaries. Then, I could see 4-5 years. But the hardware will do it in 2 years. Third party apps that were ever going to move over will do so in the next 5 years; Apple will give Intel Macs a buffer of a few more years thereafter, and then we'll get our first "Snow Leopard" type release where Intel code in the OS is dropped and macOS is officially Apple Silicon only.
Yeah, Apple Silicon is similar to PowerPC in that, with the exception of one or two oddballs, macOS is the only thing you're going to run on it, for the most part. Intel Macs were and are truly wonderful for, even despite it being Apple designed (and even Apple Silicon creeping in early in the form of the T2 chip), being x86 PCs at the end of the day. Now we're back to a relatively closed platform, and I believe that will reduce the span of usefulness of your average Mac considerably. Whereas, even though it doesn't support Big Sur (and would therefore only be supported on Catalina for two more years before losing security patch support), I can run supported LTSC versions of Windows 10 Enterprise on my Late 2012 13" Retina MacBook Pro until 2029 at the earliest. Who knows how long Windows 10 will keep supporting Ivy Bridge hardware with newer Windows 10 releases?For those who have Intel machines now - you at the very least have the fallback option to Windows and strong Linux support as well. Back in the PowerPC days those guys were in a much more vulnerable position due to the fact that not only were the processors slow, but it was an architecture that just never took off in the desktop computing world. Could apple have supported PowerPC maybe one or two more OS releases further? Maybe, but have you guys used an old PowerPC lately compared to first gen intel machines? The difference was night and day...it was better to move on when they did IMO.
Overall I trust Apple’s judgement, for the most part. There is always a case for planned obsolescence since this IS a business.