I don’t think anything assimilates religions better than Hinduism. New belief system? Just throw in a new avatar of Vishnu to legitimize it and carry on.
Even that doesn't cover it. Vaishnavism, though the most popular branch, is only one Hindu denomination. Shaktism and Shaivism don't give Vishnu the preeminant status he's given in Vaishnavism. "Hinduism" at this point in time indicates a tradition cares about some subset and some interpretation of the Vedic texts, which doesn't actually entail all that much.
That's because there's not really a there there when it comes to Hinduism. It's a nationalism disguised as a religion (by eating every local religion.)
> 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.