I don't think we are talking wipeout, but evolution and adoption driven by a self-questioning about why Allah allowed Christian supremacy, and what Muslims were doing wrong. The events that we might see in insanely-successful-Christian-attacks-on-Islam-world are:
1. An irruption of Mahdis, prophets and suddenly unhidden imams.
2. Adoption from Christianity. Some may attempt to combine Mohammed's revelation with the idea of a divine or semi-divine Isa, others may adopt Marian devotion, some may adopt Christian beliefs on expiation of sin, for example confession (we see this in OTL Shabaks).
3. Participation in Christian rituals, we saw this in OTL mixed areas of the Ottoman empire. Where Muslims and Christians lived side by side we might see more individuals who adopted an intermediate identity.
4. Pre-Islamic syncretism. If you leave a sufi school in the middle of nowhere with local tribespeople you could wind up with more things like the Yazidis popping up.
5. Innovation on what could replace aspects of Islam which were now impossible, a massive wave of innovation and reinterpretation.
It might be the case there would be an Islamic equivalent of the New Testament / Mani or early Post-Islamic analogues to Babism / Baha'i in such a scenario.
Perhaps one interesting Post-Sunni version of Islam would be best described as Post-Exilic (albeit not in the literal sense rather the triumphalist sense given the TL) Neo-Murjite Quranist / Mutazilite, possibly adopting some Shia / Sufi / etc influenced views along the way as well as embracing some limited degree of syncretism (largely Greco-Roman / Hellenistic, possible adoption of Latin / Greek script for Arabic or Arabic-derived diaspora languages / dialects, Judas/Brutus-like figures among the companions, etc) though never reaching the stage of becoming proto-Chrislamic.
Not sure how the following would work. Though another interesting ATL dominant group that would claim to be a form of Islam would be a heavily syncretic Post-Shia/Sufi sect influenced by a combination of elements from Muhammerah (Mazdaki), Qarmatian (Zoroastrian), Zindiq (Manichean), Satpanth (Hindu), Yazdânism, Barghawata, Shabaksim, Druze, Alawite, Alevi and Din-i Ilahi thought (along with other sects / schools that have not yet been mentioned).
It is also likely there would be Christian (or general non-Muslim) government censorship of Islamic texts / verses or at times outright bans of certain works (performed mainly by Muslim apostates), with Islamic religious authorities themselves practicing self-censorship out of fear of public reaction.