If a single entity holds 51% of the total mining hash power then double-spends are possible.

True, but that risk is more or less during the very initial stage, in Blockchain more it becomes older more secure it is against such threat. It is very difficult to undo the longest chain after a period of 10 years as in the case of Bitcoin. Bitcoin has grown so big that it is very difficult to launch a 51% attack at this stage.

I remember reading about a few virtual currencies that existed in the 90's. They either were not able to stop double spends or they were centralized. Bitcoin had the perfect mix of being decentralized while also preventing double spends.

BitGold tried but failed to prevent double-spending. Bitcoin gained popularity all-across because of its ability to prevent double-spending.

Thank you.

#affable