Downgrading is not supported by Apple because there are far, far more potential problems with downgrading (the inability to restore from a previous backup, thereby losing access to all of your personal data, for one).
In this forum, we only discuss what is approved by Apple. And as far as a second hand iPhone having warranty-related options? Even an original iPhone, sold on day one, still has the option of an out-of-warranty replacement for $149. However, once any unauthorized service is done on the device, that option is removed. So, the fact that the warranty has expired doesn't remove all the Apple service options.
Once you proceed past the parameters that Apple has set, you are on your own. Additionally, Apple reserves the right to refuse activation to any device that has unauthorized modifications or service. That INCLUDES iOS downgrades. Some users have reported that such devices are not allowed to be activated by the Apple servers, and that the associated Apple ID has been banned from the App Store & iTunes Store.
I simply am advising your of the risks. I'm not telling you that you can't do whatever you chose with a device. Personally, trying to 'save a few bucks' isn't worth the risk of ending up with an unusable device.