TopLought - LiveStreaming

I noticed this weekend that a few of the posts from my deck are rotting at ground level, as you can see here. My question is, would there be any reason I couldn’t cut these off just above where they’re rotting, and pour a concrete footing beneath them to anchor the existing posts to? My initial plan was to replace them completely, however after looking at it closer, it looks like doing so would involving tearing up a good bit of the deck, and I can’t even find anywhere in my area where I could buy posts this big. When I was researching how to install the new ones so they won’t rot, I came across several places where it was suggested to fasten them to the top of a concrete pier as opposed to sinking them into the ground with concrete so I’m wondering if there’s any reason I couldn’t do that with the existing posts.