Pics would help. Where are you from? If the frost line is 2.5ft then normally yes foundation and footings you would generally want them below the frost line if were talking about a homes foundation but a deck isn't going to carry as much live and dead loads.

The main thing you might have to worry about is frost heave, which is when water saturated soil expands upon freezing. Which you might commonly see resulting in damage in retaining walls. If the only thing you've seen in 10 years is loose fasteners, I think you're in pretty good shape.

Replacing an entire deck however seems a bit excessive if the footings are the issue you could technically salvage most of the deck and replace the posts and add reinforcing brackets at structural joints. Look up strong ties.

Generally speaking structure wood as long as it's well maintained and there's no insects or standing, consistent, moisture. Last a fair amount out time. I would however recommend inspecting the wood in the actual footings. Wood set into post footings underground have a tendency to rot out, no matter what you do. If that's the case grab some post jack replace the problem wood and resecure it.