That's optimistic and naive. See Intel/AMD. The problem there is that the cost of attacking the incumbent is billions of dollars, and generally you see no ROI at all until you've spent multiple billion.

Market forces can ensure that competition will exist as long as barriers to entry are kept low enough that people can afford to compete. The larger the barriers are, the more aggressively an incumbent can be abusive without needing to fear competition.

But the use of an alternative is only possible, if the alternative is on/near the same level of usablity as the product that is market leader. If it's not the case, than most users won't switch and there is also the factor of the "mental burden" to switch to a product you're not used to and stay in the "preferred loop".