Actually, it could possibly save them if they do it right. My understanding is that mirrorless cameras use less moving parts and are generally less expensive to manufacture. Mirrorless technology has also greatly improved over the past 6-8 years, and continues to close the gap in many, but not all, areas. I shoot with DSLR's and mirrorless bodies (Olympus and Panasonic) and find that each body has advantages and disadvantages. I greatly prefer focusing on static objects with my E-M1 over my Nikon bodies, as I have focus magnification and peaking at my immediate disposal. And the focus accuracy of mirrorless camera on a static objects is quite good. Remember that mirrorless cameras that use contrast detection systems do not need any type of user AF adjustments like DSLR bodies. What you see is what you should get since you are focusing off of the sensor (not unlike Live View in a DSLR).I hope this does not bankrupt them.
I hear your concerns, especially in light of how the Nikon 1 was handled, but the AF technology in that camera was supposed to be quite effective for a mirrorless design. I think they can design a good quality body that can produce IQ comparable to their current 24MP bodies, but I am concerned about how many lenses they can design and market. I suspect that if they launch a lower end model, it will possibly replace the 3XXX and possibly the 5XXX models. And if they do launch a higher end model, it will probably compliment bodies like the D750 and possibly the D850. In general, I think that the market for lower end DSLR's is starting to further shrink as cell phone cameras continue to offer more features and some marginal IQ improvements. I also suspect that the high end bodies will remain for some time, as the margins are probably better, but eventually they will become specialty products in a decade or so.If past performance is any indication, Nikon's mirrorless efforts will likely be lukewarm and handicapped in various ways by cost-cutting, lack of mirrorless design experience, or to protect mid- and high-end DSLRs. I certainly don't expect D850 performance for a long time. A new short-register lens mount allows smaller bodies, but does not necessarily guarantee smaller lenses (see Sony FE). I think a balance can be found between smaller body size while maintaining the DSLR mount - Canon's SL1 and Pentax's K-01 both achieved this without getting rather awkward like Sigma's SD Quattro design - but I fear that small size will be prioritized over high performance and F-mount compatibility. Other than body size, the primary benefits over DSLRs will be in EVF implementation, electronic shutter, and live view. Anything less than current state of the art mirrorless from other manufacturers will be a failure in many minds.