I think letting sprouts be confined to sprout parties would only work at times like now, when there are lots of sprouts to go around. But duty finder's primary purpose is just to get people into content faster. Cause if there's one thing that kills a new player's experience the fastest, it's having to do a dungeon and being unable to even find a group.
Maybe if it were an optional thing, with a convenient check-box. Then people could choose to be matched with a more similar party or just have a faster queue. I have a feeling that new DPS players would never be able to use the SBMM though. The queues are long enough as-is, and they're heavily supported by veterans who want the adventurer-in-need bonus.
I think just throwing everyone into the same pot actually sorta works for the community anyways. Because everyone quickly learns/accepts that parties are variable. If you give people the power to choose their teammates (and make it more convenient than PF), then many of them will loathe the idea of leaving their bubble and playing with different kinds of players. And taking that first step out of sprouthood and into end-game content might be extra tough. If I were still a sprout, I'd prefer to have veterans helping so I wouldn't get stuck on anything, and I'd have a good example to learn from. Maybe that's a little pretentious though...
TL;DR: Convenience and stable queues are more important than comfort. And SBMM could lead to an echo chamber effect and harsher learning curve. So I'm still skeptical. The devs would have to put extra work into the surrounding systems to deal with those issues. It's possible though.
Edit: To be clear, I admit that it would be nice for sprouts to have the option for a more chill experience. Being able to spend more time exploring a dungeon for the first time, not feeling rushed, getting to see more boss mechanics before it's over, etc. It's just tough to create a system that allows for that, while also promoting personal/community growth AND being convenient enough that it doesn't feel like a waste of time.