Just saw this comment in response to the linked article on BBC Sport:
I'm being serious here, he had better win that race or it will severely impact F1's reputation. I think it's an incredibly arrogant, selfish and narcissistic move. It also sets a precedent to other drivers that it's ok to miss races for other formula if you have an noncompetitive car. If he doesn't win, F1 will be a laughing stock. If after retirement, fine, but a supposed great? That's damaging.
I concur. I think Indy went so far down hill after the Tony George banning of Champ/Indy Cars from the 500 in favor of his IRL drivers. Since then this entire series has been pretty far down the totem pole.
BUT if Alonso doesn't win or at least make a good showing (while maybe retiring due to shunt or mechanical) this will go down very poorly for F1 and McLaren.
Fernando Alonso: McLaren driver to miss Monaco Grand Prix for Indianapolis 500 - BBC Sport