Your first priority should be to get on an android version from this decade. Lollipop came out in 2014 and went eos in 2016.
My first priority is to not financially support systems of premature forced obsolescence that has led to more smartphones in the world than people (despite ½ the world’s population having no smartphone at all). Buying a new phone just 6 years after another would make me part of the problem. I am writing this comment from a 16 year old machine that runs just fine. My AOS 5 device still uses the original battery. Only incompetence could explain inability of /software/ to outlive a /battery/.
I cannot think of a more absurd reason to upgrade a phone than to keep up with captive portals. Apart from that, I must say that I may have to argue in court soon that I no longer have access to my bank account because my bank closed their website and forced people to install their closed-source proprietary app from Google Playstore. It will be easier to argue in court that the bank’s software does not run on my phone than it will be to say I have philosophical and ethical objections to sharing my phone number with a surveillance advertiser just to open an account just to fetch software, of which the non-freeness I also object to. So I am trapped on this phone for higher legal endeavors.
When you say “this decade”, you’re disregarding the age and saying the line should be drawn at years that are multiples of 10. So a phone bought in 2019 would be “obsolete” in 2020 by your logic. Obviously that’s obtuse and reckless. I bought my AOS 5 phone new from the retail shop of a GSM carrier in 2018, 3rd quarter. It’s been in service less than 6 years.
Apple is borderline reckless and they officially support phones for 10 years IIRC. And that limitation is imposed by the business bottom line. Capitalism aside, engineers who can’t make a smartphone that lasts 20 years would be lacking in competency.
As for your liability comment. I highly doubt the vendor had any liability or or requirement to support such on old os.
Captive portals are a messy hack. You do not need a captive portal to supply Wi-Fi in the first place. The suppliers do not advertise “we have a captive portal”. They advertise “Wi-Fi”, which my oldest phone (AOS 2.3) and my Nokia n800 (pre-smartphone) supports out of the box. They still connect to wi-fi today. You might be right that a pusher of forced obsolescence by way of incompetently implemented captive portal can argue in court that their advertising has immunity to old devices, but this won’t fool engineers who know they’ve needlessly drawn an arbitrary line. If the truth-in-advertising outcome would be that their “Wi-Fi” sign has to become “Wi-Fi available only for new phones”, I would be fine with that.