I am not sure there is ever going to be an adequate replacement for Matt Cain or Miguel Cabrera on the Free Agent list even if we reduced the roster size to 20 men. In real life if a star like Miguel Cabrera gets hurt the Tigers won't be able to sign a quality free agent mid-season to replace him. They will have to use one of their bench players for awhile or else make a trade. We have the same options in a fantasy league.
You are not going to be able to snatch a stud player off the Free Agent list and insert him straight into your starting lineup very often in any fantasy league no matter the roster size, especially a dynasty league. In a deep dynasty league the idea is to build a full roster and part of that is investing in some backups to fill the gap if a star gets injured. If you opt to forego a backup player to add an extra prospect it can come back to burn you. Part of building a competitive team is strategizing the best way to use your 11 bench slots wisely so you can cover your injuries while also planning for the future. All teams suffer multiple injuries, so building a strong bench is a big key to winning the league.