Tours isn't going to switch much. It was the defeat of an over-sized raid, not of an invasion.
This is one of those overrated lines repeated
ad naseum on this forum, usually with no evidence. The fact is, had Tours been a Muslim victory, there
would have been more invasions. The conquest of Hispania began with raids, and ended with centuries of Muslim domination. At various points the Arabs were recognized in Aquitaine, so I think it is safe to say that at very
least an Arab victory at Tours would lead to a Muslim Aquitaine. I could see Neustria falling as well, and maybe Burgundy in time.
Now if Constantinople falls in the first century of Islam, then we're talking. With The City gone, the Balkans go with it. More to the point, the Muslims are at the peak of their momentum and the Western Christians haven't a single competent state between them. The faithful take the Roman roads into Italy and link up with Al Andalus through Provence, while the Franks periodically shout up a rabble of disorganized infantry to be slaughtered.
You are discounting the Lombards, Bulgars, Slavs, Alemanni, Bavarians, Avars, etc. I don't see how the Arabs will be able to just sail through every single group of these peoples with no loses or setbacks. And the Franks would
not be completely helpless against them. I wonder how Tours was won in OTL? Oh, wait. The Franks.
At that point Europe is turning Muslim. It just is. The most Christianity can hope for is a series of Ethiopia-like islands at the fringes of Dar al-Islam.
I think a better example is the pattern of conversion to Islam in Al-Andalus.
As AHP said, China is the real sticking point. Without outright Islamic conquest, you're already pretty much stuck.
It doesn't have to be conquest. Potentially a situation like Southern India could develop, where most Muslims were Arab immigrants and those they converted. If there is more trade through the Indian ocean, more Arabs might settle in China, and Islam might spread more than in OTL. Very unsure about this though.