I expect manufacturers not to offer bad options like a 150 € phone with 3 years of updates. Give me a 200€ phone with 5 years? Or a 10€ service fee per year of updates.
I'm curious how much work honestly goes into these updates, though. How many patches do they maintain on top of Android that security updates would cause them to need to fix their patches again? Or how many parts of Android are even changed by those updates in a given month? I don't really have any idea of either proportion. How much money would it require us to pool to get one month longer security updates? One year?
May be something to ask Fairphone, they seem like the type of business that might be willing to share this for the purpose of pressuring the market to offer the support after a successful crowdfunding campaign.