That's true because the iPhone 5c is really the iPhone 5 in a plastic body (and a slightly better front camera) and nothing more. It is using a SoC that is 1 year old than the 5s so of course it will not last as long as an iPhone 5s.
The iPhone 6 is fine. Apple did the same thing when they released the iPhone 3GS; doubled the RAM. The "S" was speed. It will probably happen next year but it's not going suddenly render my iPhone 6 useless. When the iPhone 3GS came out my iPhone 3G continued to function just fine for another year until the iPhone 4 came out. I expect the same from my iPhone 6.
You think they will release 6s next year with maybe like 2 gb ram?