An upgrade will impact the environment. It isn't without such impact.
And the iPhone 12 is still a very fast device on the most recent OS.
So... how about when your current phone loses access to the most recent iOS update?
Or when it slows down to such degree that it's immediately noticeable to the average person (whoever that is)?
Look, everyone can upgrade anytime, to whichever model they want. But if you do it while
Apple is still selling (or
immediately afterwards) your current model as brand-new, you need not pretend you'r oh-so-concerned about your ecological impact and the environment, let alone when it's just about a few cables.