> Just throw in a new avatar of Vishnu to legitimize it and carry on.

Except for it's never done. No Hindu believes Allah or Jesus is an Avatar of Vishnu.

It can't be. Vishnu literally means the "All-Pervading One". Allah is explicitly not Omnipresent. Jesus is Omnipresent but does not have infinite forms. So neither fit the bill. Even Buddha, who we Hindus associate as one of the Avatars of Vishnu is not the same Buddha as Gautama Buddha who founded Buddhism.