I've never heard of an airline letting you hold an award ticket for 10 days. I can understand 24 hours (like a purchased reseveration), but 10 days seems quite long.

There is a limited inventory of saver award tickets. If you don't have the miles to make immediate purchase, there is likely another person right behind you who has 20K to blow right now. The airline would gain nothing by allowing you to hold the reservation while making up the shortfall in miles.