I think it's unfair to claim "built-in decay" is unique to leveraged ETFs. This happens with all financial instruments: stocks, funds, ETFs, futures, options, etc. Any price that goes up X% and then down X%, or vice versa, ends up lower than the starting point. For any instrument.

The only difference with the leveraged instruments is the corresponding loss is greater. But that's the whole point of using leverage -- the gain is greater, too.