Your logic makes perfect sense, but there’s only one problem, there is no fixed reward to bitcoin, only a fixed supply. In theory, as supply decreases, demand increases, so no matter the exponential growth in cost, the reward will still provide a net benefit.
All monetary systems are based on a Ponzi scheme plan and all early adopters naturally benefit. The overall success or failure is all dependent on monetary confidence and faith that the currency is worth something. More people that use it, the more confidence it gains, it’s a snowball effect. More people that use it, the less the supply (fixed) and higher the price, which again, isn’t fixed.