I would expect macOS for Intel machines to be supported for at least 5 years from the last date of sale of the very last Intel Mac. That would mean at least 4 new macOS versions that support Intel after the last Intel Mac is sold. The hardware will, of course, last even longer.
I do not expect Apple will shorten the useful life of Macs by switching to Apple Silicon. They may indeed bring out new chips more frequently than Intel has, but the shorter useful lifespan of smartphones is driven by factors that don't apply to the far more mature desktop/laptop PC market. PC-buying businesses and individuals are not going to embrace the idea of replacing their PCs every 2-3 years, and there aren't enough features to be added to lure people into buying more frequently - "improved/more cameras" (other than moving the webcam from 720p to 1080p) isn't going to sell a whole lot of Macs. Apple will almost certainly introduce features that aren't provided by Intel machines - that's one of the key advantages of producing their own silicon. However, I expect we'll see many of those features come with the first generation or two of Apple chips, in order to make the "Apple Silicon Difference" obvious (Face ID, for example), rather than trickle in on a gradual basis.