Even if its divisible, a total coin is 'valued' at $5,000 in your example. Just like $1 is divisible into 100 cents. The total market cap is infact what the market and the world values a total project/supply at.
Circulating supply is not a useless metric, neither is total supply. The more scarse something is, the more likely it is to be valued higher. You bring on more supply, even with static demand it is going to be worth less in totality and market cap. You seem smart enough to understand that. Everyone seems to love bitcoin and its 'finite' supply, its the reason the price is so high.
You're argument is like saying the AUD doesn't fluctuate against USD (or any other currency) based on economic activity and other variables, one including whether or not there has been a heap more of it printed via quantitive easing. The more of something there is, the lower the price although that doesn't effect the clearest indicator being market cap.
