It is very speculative to wonder whether Christianity would not be forgotten had it not found such a convenient foil in Islaam to reinforce the perception that Christianity "must be" significant (or else Islaam would not insist on defining itself in contrast to it). Still, I can't help but keep returning to that speculation. Both faiths seem to me to rely quite a lot on each other in order to seek justification to their continued existence.
I was thinking the same thing, considering how long the two religions have been at odds with each other. Especially along the front line areas between Christianity and Islam, Christians have been much more fervent and fanatical in areas where they felt most threatened (such as when Spain was invaded by the Moors, or in the Balkans which were invaded by the Turks).
Those Muslim incursions also forced Europeans to search for new trade routes to Asia, which is how Columbus and many of the early explorers came on the scene, leading to the age of colonization. As a result, Europeans acquired more territory, resources - and eventually surpassed the Muslim countries in terms of economic and technological development. This ultimately led to many of these same Muslim countries falling under the thumb of European colonialism and imperialism, especially during the decline and fall of the Ottoman Empire. So, in addition to enmity over religious differences, the Muslims now have a rallying cry which sets themselves up as "liberators" against Western "oppression" and "imperialism."
The real irony here is that, if the Muslims had simply left Europe alone and continued to allow trade with East Asia, then there would have been no incentive for Europeans to seek out new trade routes. It's likely that the relative position of Muslim nations would be far better today than it is now, if they truly acted as the "religion of peace." But their warlike aggressive incursions into Europe eventually came back to haunt them.
So, I think you're correct. Christianity and Islam somehow fed off of each other and become stronger as a result. I have seen this become even more prevalent in my lifetime. When I was a kid being raised as a Catholic, very few people cared much about Islam or worried about it. The big fear back then was communism, and the West was willing to side with any nation or regime as long as they weren't communist (which is how we got hooked up with the Shah of Iran and the Saudi Royal Family, among others). It wasn't really until the Arab Oil Embargo and the Iranian takeover of the US embassy that Muslims started appearing on our radar as a potential threat.
It's also interesting to note that, during the 60s and early 70s, the US was in the process of massive political changes and reversals of policies which existed from the very beginning. We were reforming and reproving ourselves over our past. The civil rights movement was moving ahead with full steam, and there were strong voices against militarism, interventionism, and policies commonly associated with what is known as "Western Imperialism." Those ideals and their advocates started to lose steam as threats from the Middle East became more noticeable. Consequently, as Islam became more radical, aggressive, and hostile towards the US, the US responded by electing Ronald Reagan and supporting a resurgence of US militarism and interventionism around the world.
To try to tie this in the topic of the OP, I suppose the case might be made that Islam may be worse, if only because their aggression towards Europe is what pushed European aggression, colonialism, and imperialism which some people consider harmful. If they had just left Europe alone, history would be much different.