I agree with most of what you're saying but...
This isn't just a security update, it's a full OS upgrade and these should be rolled out with far greater care as there's far more issues that can be introduced with it and in the scheme of things, the risk level of remaining on 14 with a stable OS and the latest security patches is far lower.
Historically Samsung has always been far slower than other manufacturers at pushing out it's OS updates, I'm guessing as they don't make any money from it the put less resources into getting the updates out (we could pretend they're more thoroughly testing them than other manufacturers).
At the end of the day, you're phone is still secure even if it's an OS revision behind. Android feature introduction and adoption has been slow for years anyway, is there anything in particular you're looking forward to?