Its not about number of coins per block. The 1 block/10 minutes is designed to increase security.
It has nothing to do with security. To reverse part of the chain, you always need to do at least as much work on average as was put in the part of the chain you're trying to revert, whether that is 1 block or 1000 blocks.
The 1 block per 10 minutes rule is a compromise between usability and chance for stale blocks because of network delays.