They'll be good for at least another year in terms of major updates, and I imagine they'll get minor releases right up until iOS 15 (conveniently, two years from now).
Thanks. I should have just got them the 8s for a little extra a month. I came from LG/Samsung, and I praise Apple for supporting older devices for quite some time. After two years any Android phone I had seemed like it was no longer supported. There are things I miss from Android, but this is one thing I love Apple for.