Except it isn't stupid, it is responsible from a security, engineering, and support perspective. In terms of the security aspect, like I mentioned above, iOS is used on over 2 billion devices. That is a major target for black hat hackers to want to try and exploit which could negatively impact iPhone users. Supporting many iOS versions would add unnecessary stress and load on Apple engineers & support advisors such as Geniuses. And it would take a lot of time away from Apple's engineers' work on newer iOS versions. iOS 17 for example is supposed to be a release focused on major stability and performance improvements alongside some long-requested features, which would probably benefit a lot of users. Allocating engineering resources towards maintaining security for old versions of iOS on devices supported by the latest major version of iOS would be a waste of R&D money. Not to mention if users stayed on old versions forever, what would be the point in Apple continuing to maintain iOS at all?
Regarding your last sentence, I'm genuinely confused by what you mean by that. Compared to today's iOS releases, iOS 6 was severely limited in capabilities. No app developer could do today what they could back then on new iOS versions, and regular users don't have the capabilities and features they did back then either. Shortcuts? Nope. Siri? Billion times worse compared to Siri today. Apple Maps? That too was so much worse back in the iOS 6 days compared to now. Not to mention the thousands of additional features Apple has added to iOS since 2012 and now. So I genuinely do not understand that last part of your message.