FWIW, "desktops" (by which I assume you mean Desktop PCs) went through a period where they grew by three orders of magnitude in CPU efficiency (4.77MHz -> 3GHz plus caching/pipelining), RAM capacity (< 1MB to > 4GB), storage capacity (< 20MB to > 2TB), etc. It used to really make a huge difference to buy a new Desktop PC every year or two. Now it really doesn't make that much difference; Desktop PC hardware has been basically stable for the last 5 years. Phones, primarily due to their battery-induced and size-induced limitations are less far along this growth curve thus far.
That said, I have a 2.5 year old phone which is still getting vendor OS upgrades, including two major OS version upgrades in the time I've owned it. (No it's not Android based. Granted I did deliberately buy the highest end model available at the time. And they are still selling the same model now, as their lowest end model, which probably helps. But it has received new OS features every year, even if not all the features are available on the older hardware design. Showing that given some planning/effort it is possible to provide a "cut down version" of new major OS versions on older hardware designs.)