Even if Rielly becomes, say, a top 15 d-man (which is a bit of a push), the Leafs are going to need more guys at that level or close to it. It's not a question of whether a problem exists or not. It's the backbone of the team.
Shanahan could have a meeting with his guys next summer, and decide to go acquire a #1D-man via trade, and still not get a suitable deal in place for years on end.
This is very true. Teams are very reluctant to move #1 defensemen, and there are usually underlying reasons for such a move(salary, retirement contracts, attitude problems). You could offer the moon, and still come away empty-handed.
The best approach is drafting, or free agency, and both come with risks. With drafting, you need to be a bit lucky. Or a lot lucky. You either need to be in a position, and at the right time(because they aren't usually available), to draft that sure thing. Doughty or Ekblad come to mind. Or you need to be drafting high enough, and be lucky enough, to get that highly touted D prospect who is one of the prospects who hits his potential, and that's more uncommon than people seem willing to admit.
The other option is free agency, and that involves a combination of being a favorable destination, and paying a crap load of money to said free agent. If you aren't a favorable destination, you should probably expect to pay even more. That contract could end up biting you in the ass, because it puts you into a position to sacrifice depth to make room for it, either in the present or future. There's also the risk the player doesn't live up to that contract.