I see lots of confusion here. Android IS NOT Free/Open Source software, at least not the one you get with your phone. It's true that the bare bones OS is FOSS, but the same is true with iOS and Darwin. But they have a lot of proprietary crap on top of them.
The reason for offering a longer or shorter period of OS updates is a business decision, not a technical matter. There's at least one android phone (fairphone that I know of, not an ad or anything) with 7 years of updates, which is longer than the iPhone.
These arguments of costs of development and having to support hardware fall apart when you take into account that some smaller vendors are offering longer support than bigger ones like Samsung or hwawei, and when you look at which hardware they mount, it's basically the same. Same processors, very little variety in cameras and screens...