What property guarantees the continuing profitability? If the cost of electricity triples in 20 years, the number and size of transactions wouldn’t change (as far as I know), but running a node would become more expensive and possibly unprofitable.
What stops someone from creating a client that will send repeated transactions just below the fee maximum when it has to in order to avoid fees? Or from many different identities to the same one (if this is possible)? I can see there being a great demand for such a client if BitCoin becomes very popular.