There will come a time when Brady's skills will erode, to the point he will need to be replaced.
Nobody knows when that day will come; right now it is pure speculation.
If Brady does not recognize it when that day arrives, Belichick will pull him aside and let him know. The end of Tedy Brushi's career would be a good comparable.
In the meantime, based on Brady's performance in the most reason, he is still the better choice for the 2017 season. One can debate on what the best long term plan may be, but I do not see how there is even a question on what is best for this upcoming season.