It is done throughout professional sports. No reason why it can't be done here.
Just get an idea if they're open to an extension. There will be your answer whether to do it.
No one is open to an extension, particularly at this talent level, no one.
One thing to consider is a renegotiation of contract + extension afterwards, but that requires cap space (and I'm not up-to-date on all the rules that go with it).
That said, even with the above scenario, none of these mega stars are taking a non-option year after this season. Next year is when contracts will be at their highest between these 3 years. That's why Durant only went with 1 + 1, like LeBron is doing as well, taking advantage of big cap jumps to improve their MAX.
History, throughout all of professional sports, suggests that you are quite wrong. Mark McGwire, Matt Holliday are just a couple of quick examples from baseball that come to mind.
However, if Westbrook and his agent say flatly, "We're not going to resign with you," then you don't do a deal.
What I have seen about his preferences, however, suggests he would be quite open to Boston. If the answer is, "We would certainly consider an extension with you," then you make the move. He won't sign one this year, for sure. But that historically has not stopped major moves in the past - if the player is open to an extension at year's end.
I don't expect that Danny is pursuing Westbrook, however. It's Cousins.