I am all about diversity.
Well, I try to be.
I think there is a little anti-Islamic feeling that keeps popping up. No-one seems to complain about Christianity suceeding where it did not traditionally (Mongolia, the Levant, Qarluk) but people seem very sensitive to Islam doing well in areas it did not historically.
Not that this should be a balance sheet, but lets consider:
Where Islam has been rolled back.
1. The KoJ are a reasonable starting nation and control 6 provinces, including Jerusalem, the third holiest site in Islam.
2. Qarluk is 'nestorian', where it traditionally was Islamic. Bit of a backwater strategically, but there it is all the same.
3. No Mughals. Yes, the Mughals did not convert a lot of India to Islam, but it was an Islamic power which our game does not have.
4. No Ottomans. Islam's big power IRL is replaced by a resurgent (Christian) Byzantium and disunified Turkish minors. Hell, the Byzantines can even get Turkish culture.
5. A more significant religious split than the sunni/shi'a issues of vanilla.
In exchange, Islam gets:
1. Tech increases for Caliphate and Mamelukes.
2. Champa (assuming this gets written up)
3. Al-Andalus, which doesn't have an easy run, but does get to colonize.
On the whole, I'd say Islam has a negative on this balance sheet, even when one considers the value of the tech increases.
So, having Islam be able to make advances in other parts of Asia is both historically plausible and not - on the balance of it - unreasonable.
I like the idea of having Christianity and Islam permeating Asia, as long as we have:
1. The major Asian starting states are Bhuddist/Hindu/Confucian
2. There are reactions against new faiths.
Just my two cents worth.