What yesterday's game shows is you don't need a closer if you can't hold a lead in the 8th. Certainly there is something to be said for a proven closer but those guys also got developed originally. If you've got the right young guy a Kimbrell, Jansen or Giles is just as good if not better than signing a pricey FA headed for decline. I think the trade market is probably a better way to go (Giles aside, they generally don't cost that much).
I'm not sure SD has actually been overused, we just assume so because he's struggled. Given his age, experience and starter pedigree it probably has been too much to ask of him, but who knows? Right now he just doesn't have it though. Fatigue might be part of it but he's gotten a lot of time off in July and August. He would be so good in that Andrew Miller type role though if he can do it, not pitching on consecutive days but capable of giving you two innings 50% of the time. That's what he looked like he was going to be when he first came up. So in that sense, sure, a different closer would be great. And adding another talented relief arm certainly wouldn't be a bad thing.
But this was a choice Klentak made. Let's ride these starters and reliever and see what they are made of. They might be good enough. If they're not, it's a learning experience and you've got more info with which to decide whether to upgrade this off-season.