A “cartel” is simply profit sharing. The reason that competition per se exists is obvious. Some know that they can take it all and some know that they can lose it all. Only losers are interested in sharing for greater material gain.
What does it mean to doubt capitalism? If it means doubting its perfection, then that’s not even up for question. No human thing is perfect. If it means entertaining the notion that anything salutary can be done about any imperfections that may occur via a market process, then the other side of that coin is a necessarily concomitant faith in the efficacy of some non-market action in remedying that imperfection without creating worse ones. So then you have to ask yourself what is so meritorious about that given non-market action to warrant such faith.
I don’t expect perfection of capitalism, so I don’t “doubt” it. I’m not so much a believer in capitalism as free, voluntary interactions, and there is little that can shake my support for such an ideology, i.e. succumb to the notion that people are too “stupid” for their “own good” etc.
With respect to economics/praxeology, I always keep a reasonably open mind, because like an knowledge-seeking endeavour it can be expanded and at times, revised.