You would be hard-pressed to find too many people involved in MotoGP looking forward to Valentino Rossi's retirement, whenever that may be. It's still at least 18 months away, and might not even come when Rossi's current Yamaha contract concludes at the end of 2018.
Rossi being Rossi, you wouldn't bet against him racing beyond his 40th birthday in February 2019. So it may seem a little early to cast an eye towards his exit, especially if he maintains the form that has him leading the 2017 championship after four races.
But if this does end up being Rossi's last Yamaha MotoGP deal, whose number gets stuck on a factory M1 when the yellow #46 comes off at the end of 2018? If Yamaha had to make that decision any time soon, I reckon a red and white #5 would be the one being printed.