Funny enough, for new hires on their first bid, they are tending to get DTW and ACY, but those are "placeholders" if you will, since they're still based in FLL until finished with ground school. By their second bid, which is usually close to the type ride/beginning of OE, they're getting their preferred base withy maybe the exception of LAS. The newhires into the other bases look like its for continued expansion.
I guess a real quick and easy way to take a look is to see where you fall in relation in the other bases on the system bid. And doing it that way, you may be right about DTW being junior. For instance, I'm 1/3 in FLL, DFW, LAS and ORD, 1/2 in ACY, but 1/4 in DTW. So maybe you're right about DTW now being the junior base.
Either way, I don't think anyone looking to come to Spirit needs worry about basing, since it's not unexpected for any base to be readily attainable for new FOs.
Captain slots on the other hand....