It’s cause they know that most users are ok using the old stuff and want to force people to update. The iPhone 5 and 6 were fast enough for most needs

They’ll definitely keep dropping at least one iOS every year on average. Right now the 6S is going to be 4 years old. If they cut back to only 3 year old models. That’s pretty recent. Will be interesting to see if they do that. Removing the 6S Plus is also interesting. I thought maybe they’d give the Plus or higher end models a year longer to incentivize purchasing those.

t’s not really a huge deal to me that they’re likely purposefully going to try to keep the newest iOS limited to only a few years. Supporting 4 year old devices is still pretty good. And being on an old iOS for a few years isn’t the biggest deal.