It would push hardware manufacturers to produce good reliable hardware instead of 10 crappy new phones every year, and to partner with reliable systems developers/vendors instead of pushing their own broken-in-1000-ways Androids.
Also bringing up the original iPhone is a strawman, as that's not going to be supported by this law any more than it is today, newly released phones however are. And since technological progression has slowed down a lot in the last few years that really shouldn't be that hard.
No, but most people also don't need SoCs which performance doubles every 2 years. If the software was properly written and optimised (which would be more common if the firmware developers expected the hardware to have a long lifespan), there would be no reason any phone manufactured in the last 7 years to be sluggish. Anyway most people just want their basic apps to work (e.g. phone, emails, whatsapp, agenda, hotel booking, photos, maps, chat, music player, etc.), none of which should require a very powerful CPU or tons of RAM...
Why? My Nokia N9 is still my daily driver. Only the lack of software updates is making it less and less useful.