They don’t mean sensor reliability. Pretty sure the issue is that the current top dog in the Hall Effect space (Gulikit) has subpar quality control. If you luck out with your order you’re set for life, yes, but plenty of people are reporting quality issues with Gulikit outside of the sensors themselves - ie stick feel, tolerances etc.
It’s almost the exact same performance profile - calling it a Deck 2 would be borderline consumer fraud. But more importantly, Valve want to be very consistent in their communication with developers: just like a console, all Steam Decks are designed to be almost interchangeable from a developer perspective, with only slight differences at most. When Valve release a device that developers need to target differently from the current Deck, that will be the Deck 2.