"Why is purchase chains not the best security?"

The number of people & companies that can buy that entire coin's supply/market cap is huge for almost all cryptocurrencies.

Heck, there are companies whom can buy over half the crypto's w/out hurting their wallet!

Microsoft has over 130$billion in CASH & lost 232 billion in value in 2019. If these giant conglomerates wanted to buy your purchase only coin, it wouldn't be problem and if the 3 top companies with cash on hand got together, they would have over 300$ billion in cash to buy all of the cryptocurrency market!

https://www.pscp.tv/w/cXdlJzFsWkVwWVdwTnJsUW58MVlwS2tRWWx2QkVKag3lstUQbO5fXW6bJmgXLoEbnKkjjNQr9hYOLaWuNl_l