In this case I think Android 5 was 2014, so almost 7 years old a this point.
Amaysim are pretty good, Though if you can convince them to add a security note to your account its worth it (Colleague of mine had his mobile number hijacked while on holiday and they used it to access a few of his online accounts)
Well, Android 5.1 was EOL in 2015, so you willingly bought an unsupported model. I'm not saying Android has any sensible long-term support (in fact, I spent the last weekend installing LineageOS because my 2018 phone doesn't have support anymore), but this instance is hardly Google's fault.
I think most people on HN are comfortable doing a bit of research to see when a device will stop receiving support, but I don't think that's reasonable to expect from everyone.
If I walk into a shop and buy a phone, new in the box, it seems pretty reasonable to assume that it's operating system will work with whatever apps I install from it's built in store, and that it will receive security patches for at least a few years. That seems to me like a pretty low bar, but it's absolutely not the case in the Android ecosystem.
I said prepaid, but as I did it it’s actually postpaid but with an initial $10 prepayment required. Ah, good times back in 2014–2017, getting those bills for less than a dollar (commonly 12¢) every quarter. Then I moved to a tiny country town where an Optus tower a few hundred metres away became my best option for internet, 50/25Mbps and far more solid than any NBN anecdote I’ve heard.