There are some very powerful apps available for iPhone/iPad, maybe the limitations don’t actually limit developers much?Devs can’t get around limitations of the OS. Or at least not without Apple stepping in and punishing them.
As for sticking to an older phone, there are plenty of reasons to get a newer phone besides raw processing power. Camera and display improvements, battery life, etc. my 7 Plus does everything I need it to, but I do still wish the camera was a lot better.
Towers will always be more powerful than all in ones, which are more powerful than notebooks. Then there are tablets, then handhelds, then wearables. But in a few years handhelds will be as powerful as notebooks are today.
re: phone features, if you want the latest camera, you have to buy the latest phone. But if it only has the processing power of a 3-5 year old iPhone, you’ll be severely restricted with respect to what apps will run well. But 3-5 year old apps will still run fast.