To be fair, it's been this way with mustang for decades. A big design change, followed by another generation with similar styling, so two similar back to back generations, then another big leap in terms of design. It was that way with the mid 90s and the new edge mustangs, the '05-'09, and 2010-14 cars, and now s550 and s650.    My guess is whatever follows s650 will take another huge leap in design. 

To me the Cybertruck customer is more the full size suv crowd than the truck crowd. 

New design usually attracts buyers, but after initial excitement, cost/value plays a major role as we all know.  IMO data in link below addresses your question for the most part.  Comparing the Mustang GT Premium Fastback over recent years it appears higher prices correlate with lower sales.  I’m not questioning whether Ford should or should not lower prices, or even if they can.  My personal preference is that Mustang not be allowed to become irrelevant or worse extinct.   https://www.cjponyparts.com/resources/mustang-prices-through-the-years

Lightning was more successful even though it sold in a relatively low number as it is related to F-Series while Cybertruck is a bespoke vehicle with little to do with the rest of the Tesla line. Rivian is starting to turn a profit at the lower numbers.

Henry? 😀   In that case better find a way around CAFE limitations so selling higher volume makes sense.

It’s not due to CAFE either according to Said Deep (it’s due to supply constraints if you recall).  Folks can speculate all they want, but Ford would gladly sell every Mustang (Mach-E included) they can build.  There are many factors for the decline in sales for an “all new model”.  

FYI...they just trade marked "BOSS"...........

Those Mustang buyers pretty much know what they want and apart for the new Ecoboost models, I don’t think Thers much that could be done with Mustang.   Now if Ford was to consider building another car at Flat Rock, maybe the C2 Zephyr? Just a thought that links back with a recent discussion - you may/may not agree with above as it may not add enough volume to justify inclusion which is another issue…

Styling changes wouldn’t make much difference.  And they already have the best powertrains so nothing to change there that would help.  It is what it is until Ford drops pricing.  But as was stated previously by morgan20 - they probably don’t want more sales due to CAFE.

I have no proof but something tells me that no amount of change would pump prime Mustang sales. If Ford wants to keep it around, they really need a second vehicle at Flat Rock to keep enough volume for at least one shift.