In terms of apps and support, no it won't go downhill. I would be extremely surprised if Microsoft didn't honour their existing support commitment, and WinRT is WinRT regardless of whether it's ARM or x86.
You're getting Windows RT 8.1 and the app situation will only get better. This is absolutely not the same as a vendor having a fire sale of a device on a sinking platform.
An app on the marketplace doesn't need to work on RT, I can really see and imagine X86 tablets being the main thing for Windows, I mean really, how and why would people buy RT tablets when it's looking like much faster X86 tablets are coming in at the same price point (As is currently)?
My fear is that W8 tablets for X86 really kicks off and then we lack RT ports (But that's just a fear, it might not be the case, and then the X86 tablet sales would only bolster the RT apps too), Blazblue I'd love to play on my tablet, but it's X86 only.
EDIT : Bearing in mind, X86 doesn't really need apps as they can run anything on Windows.
EDIT 2 : And I'm a pessimist.