So can we really trust it's conceptual existence?
That's generally a fascinating topic, although i'm quite confused what the argument is here :-) can you explain again what you mean (like i'm ten years old)?
That's akin to saying there's no objective truth- a contradiction.
I would add that "truth" also depends on the person and the circumstances. On your point about time, "the earth is flat" vs "the earth is a globe" is the perfect example of how truth can change over time based on data and info at this specific moment