I seem to remember a figure (forgotten the name) who emerged at the same time as Aquinas, who preached the same things as Aquinas with regards to Islam. He ended up being executed for heresy.
The Islamic Golden Age was great for the time, but we need to remember that in comparison, Christianity was in its dark age. It's not as if Islam went down-hill per-say (OK, it did with the expansion of the Ottomans after the Abbasid empire, but that's more of a criticism of the Ottomans than anything else), more that it really failed to progress from its golden age, and became outdated, and retrograde.