I think you do not understand how it works. Hardfork does not alter the maximum supply of bitcoin, it just creates an altcoin based on the current blockchain. But bitcoin will still have 21M maximum supply. If ever the hardforked coin survives, it is just an altcoin and will compete with bitcoin on the market.
You are absolutely right. Bitcoin can't be created more than 21 million. Only the price of Bitcoin will continue to grow in the future. Hovewer, I can't say the same about other cryptocurrencies so their diversification and quantity may grow.