You stated, when was the last time someone like Crosby was traded. I never said he will leave, or will get traded. I said should. Since you focused on Crosby level talent, let's look at the other big 4. I consider Crosby #5.
What's so objectionable about the question?
Wayne Gretzky was traded in 1988. Arguable at his peak. Edmonton won the cup the next year. A couple years later, they traded another cornerstone, Messier. They missed the playoffs for the next 4 years, but rebounded nicely towards the end of the 90's.
In 1970/71 Gordie Howe left the Red Wings with nothing in return. They made the Playoffs once in the next 13 years.
Bobby Orr was done with the Bruins in 76 when he walked away with no return. The Bruins maintained a very good team despite losing him for nothing.
Finally, Mario. Penguins had a nice run, partly due to Jagr and their depth when Mario left in 97. When he returned ironically, the product slid and the team was on the verge of folding before the luckiest lottery bounce in league history went their way.
Is it an exact science? Nope. Could they do fine, if Crosby and Malkin and Letang walk for nothing, sure. Do I think they will? No. I think they will be more like the Redwings from the list above.
The question is long term success or short term success.