Add to that the fact that Berlanti is on a huge deal with them and WB, and I would find it incredibly hard to imagine that they'd even mention the word "cancellation" with regards to Supergirl unless Berlanti specifically isn't opposed to it. They're not going to risk pissing off the EP of basically what amounts to 80% of their top shows on the network.
Or that Melissa Benoist decides to end the adventure when her contract of 6 or 7 years will be over. She will have been at the end of her engagement so the production couldn't blame her leaving the show still in progress With a very young child to care and a husband working on the East Coast of USA, maybe her priorities will change and after to have had her other half supporting her for years, she would want to reciprocate the gesture (especially if his show works) for a some time before finding a new acting job.
And then, it is also important too, to know when and how
to stop an adventure like a show. Often, years after, the producer(s) and actors admit too often, not having been able to stop their shows in time in order to not annoy their fans.
IF Berlanti really respects Benoist (I'm not so sure since s3 according to bad work made but with this season, they reach a new level!), I hope that he will reserve a beautiful closure to the adventure of Girl of Steel.