The problem of course is the payee can't verify that one of the owners did not double-spend the coin. A common solution is to introduce a trusted central authority, or mint, that checks every transaction for double spending. After each transaction, the coin must be returned to the mint to issue a new coin, and only coins issued directly from the mint are trusted not to be double-spent.
The threshold can easily be changed in the future. We can decide to increase it when the time comes. It's a good idea to keep it lower as a circuit breaker and increase it as needed. If we hit the threshold now, it would almost certainly be some kind of flood and not actual use. Keeping the threshold lower would help limit the amount of wasted disk space in that event.
The problem was, each thing required special support code and data fields whether it was used or not, and only covered one special case at a time. It would have been an explosion of special cases. The solution was script, which generalizes the problem so transacting parties can describe their transaction as a predicate that the node network evaluates.