inflation disagrees with your premise, there is no reason why number can't always go up, there are plenty of countries that prove otherwise.
Physics and reality do not. Once the number gets big enough it loses all practical value. Humans are not really built to reason about very large numbers. At some point you reach Zimbabwean inflation and your currency loses all value because everyone is paid in the trillions. Then you reset your currency back to lower numbers because that's what humans and their systems can handle.
So, no, the number cannot always go up. There is a practical ceiling beyond which numbers lose meaning and value.
Heck, the dollar is already it. What used to cost a penny now costs ten dollars. We just think in dollars and forget about the pennies.