As for cost, I don't see why it would have to go up all that much. Apps are already upgradable on phones and much of the OS is hardware independent. So the only real pressure point is with the kernel and other hardware dependent code.
(1) Battery stops holding a charge
(2) The device gets damaged
(3) Cameras get a lot better
The Nexus 6 automatically throttled performance based on battery left, but at some point the battery wore out to the point that less than 1/2 an hour of use got you below that threshold. After that the phone was very laggy and frustrating to use. No way anyone would want 5 years of that experience, updates or not.
Not even going into the problems with second-hand phones and that poor people de-factor have zero legal rights as they don't have the money to take sellers to court.
Many used phone sellers/marketplaces offer extended warranties on second-hand phones, which risk-averse buyers should purchase.
This thread is about EU law.
In EU you don't have to take sellers to court, you just have to nag customer protection authorities until they do.
It might take some time: Google still hasn't gotten a massive fine for abusing its position in search and ads to kill competing browsers despite my reports but I will not be surprised when it happpens.
PS: come on guys and gals and do write to your local competition authorities. The sooner we can get this sorted the better.
The result of this law would be that cheap phones will get more expensive for no benefit at all and expensive phones will cost the same.