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You are absolutely right that Bitcoin total supply is key but it is not only thing that sets its price. If its supply were suddenly much bigger its value would drop significantly from today levels which are based on its fixed 21 million coin limit.
Predicting Bitcoin future price just by looking at its supply is wrong because other major factors like how much people want it. Its usefulness and how many people use network and its security and also government rules new technology, wider economic conditions, and even scheduled halving events all play big role in shaping its constantly changing value.
If only supply matters so their are many coins and tokens which have less supply as compare to Bitcoin supply but they are still low in price.
I also agree, if the supply of bitcoin was larger, the value of bitcoin would probably be significantly smaller, but the thing to keep in mind is that the price of each bitcoin could be lower but the capitalization could be the same as it is now.
The price of an asset is determined by its use case, utility, demand…combined with supply, and Bitcoin is no exception. Therefore, if we want to price an asset, we need to combine those factors together, we cannot price them based on only one supply factor and ignore the remaining factors.
Many centralized coins have limited supply but cannot be as valuable as bitcoin because they lack utility and real use cases. Or gold has a very large supply but in return it has too many use cases and practical applications...so its value remains high and increases over time.