Part of me thinks that they'll be dropping a model each new Apple Watch with the iOS releases, like how the iPhone 4S got cut out with iOS 8. But in order to get as many Apple Watch users as possible they might still keep it. Also, think of it this way. The iPhone 5C was released as a brand new phone only 3 years ago when the Watch gets released unlike 4 for the iPhone 5. So since it's still a decently new phone Apple may still want to support these users. And the 5C has same specs as the 5, so they would support both.
The 4s doesn't represent any kind of precedent because it was the first release of the watch, and in all likelihood a marketing decision more than a technical one, to encourage upgrades, and leave 30-pin phones & 3.5" displays behind as fast as possible.
Since then Apple has learned demand has not necessarily been as strong as they hoped. Dropping a supported model, much less two, while trying to grow the market for the watch, especially if the 7 proves as uninspired as it looks, would just be a bad idea. As of April, 60% of all iPhones in service were 4" displays. A fair chunk of those are 5 & 5c models. While a lot have probably upgraded, or tried to upgrade to the hard to get SE, it seems logical. There's still a strong base for the older models in place.
The Watch also adds Pay to those older phones, and that's something Apple Also wants to encourage. If 5 & 5c customers hold onto their phones another year, and a new watch comes out, why would they buy the old buggy technology, just because their phones won't run the new?
It could go either way, but my money is on it being supported.
