Unless your phone is showing signs of hardware problems, I would recommend using the phone a while longer. When the new phones are released in October, you can upgrade then.
If you don't want to spend money on a new phone right now, get a battery replacement from Apple ($80.00).
I had battery problems with my iPhone 6. Did several new install setups and limited what I installed. It would die after 4 or 5 hours of playing music already loaded on the phone. Had the Apple Genius take a look, and after running tests on the battery, he said it was probably a software issue, as my cycles at that time was only 362. He offered to replace the battery, but he thought I would be wasting my money if I did. I left the store with the phone as is. After 2 months, my contract with Sprint ended. I return my phone to Sprint, jumped ship to Verizon, and got an iPhone 7+ It has been smooth sailing on the Verizon waters thus far. If my contract had not been at an end, I would have gone for a battery replacement, as many i6 users have had battery issues, even though Apple hasn't made a direct statement about the iPhone 6 battery.
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In my opinion, you are giving out terrible advice (re: not upgrading the OS; Apple purposefully crippling the phone with the OS).
1. Keeping up to date with OS updates helps to keep your phone secure, helps to correct bugs and other problems, as well as helping the phone to stay compatible with other devices and programs.
2. There is absolutely no credible proof to substantiate your claims.
3. There are a lot of things that can cause a 2 year old iPhone to slow down. Just because a phone slows down doesn't mean the OS is at fault nor does it mean that some sinister Apple motives are at play.