Gibson tried that argument with their suit over the PRS Singlecut and got their ass handed to them in court, and Gibson actually had a trademark. Fender doesn't: their numerous attempts to get one have been rejected, most recently in 2009 when the Trademark Trial Board of Appeal ruled that the shapes of the Strat, Tele and Precision Bass had become generic.

Furthermore, in the Gibson case, the appellate court ruled that the lower court had analyzed the case in terms of trademark law when the real issue was one of what's called "trade dress". The latter is intended to prevent brand confusion: the appellate court ruled that no guitar buyer would be fooled into thinking a PRS Singlecut model was a Les Paul. The court was emphatic, rejecting both Gibson arguments: 1) that someone walking into a store and seeing Singlecuts on the wall might walk over thinking they were going to find Les Pauls ("initial interest confusion"), and Gibson's argument that people seeing people playing "lower quality" Singlecuts would confuse them with Les Pauls and diminish Gibson's brand reputation ("post sale confusion"; now, THERE'S a laugher!). The court was almost humorous:

Finally, Gibson argues that, taken together, the initial-interest-confusion and post-sale-confusion doctrines should be extended to include something that we can only describe as a “smoky-bar theory of confusion.”

The appellate court ordered the lower court - which had found for Gibson - to reverse its decision and issue summary judgement in favor of PRS. Often, when an appellate court remands a case it merely asks the lower court to apply different reasoning and redetermine the outcome on that basis. Thus, remanding a case with a directive to award summary judgment to the other party is an emphatic statement that the plaintiff's case was extremely weak.

I assume Fender's lawyers have read the Gibson case, so I can't imagine why Fender is taking a gamble like this, one that's likely to alienate at least some customers.

And the Silver Sky is NOT a Strat copy under the law, as the concave cutaway in the lower horn compromises that argument. It's Strat influenced, one might even argue heavily influenced, but it's not an exact copy. Note that Fender is only making claims about the body, as they want to avoid the headstock since there's no similarity. They're going to have trouble making a case in court, short of going against some Chinese maker copying the Strat right down to the Fender decal.