Bitcoin is the incumbent in the space with a strong lead. It is not feasible to create faster blocks than about 60 seconds (as short block intervals create a host of other problems). 60 seconds however, is still way too long to wait at a cash register for a payment to confirm, and would be incompatible with current checkout processes.
- Confirmation times cannot be short enough in another crypto currency to make waiting for confirmations feasible.
- There are payment processors in Bitcoin that assume the risk of accepting zero confirmation transactions, while this is much less common in other cryptocurrencies. The risk is somewhat calculable, by assessing priority and other features of the transaction.
- Retail payments is not where Bitcoin will break into mainstream, as it is the area where it has the least advantages.
Right now, zero confirmation payments are possible through payment processors. Later, when "Bitcoin's popularity is high enough and people come up with services that try to do massive double spending", other services will be made use of to curb it. If and when this becomes a huge problem, e.g. multi-signature payments could be used more to change incentives.