It also makes sense for a company to reduce the number of different makes and models of aircraft so that a pilot can move from one to another without too much retraining, so they can reduce the size of spare parts inventories, service more aircraft at fewer locations, stuff like that.

And using different vendors is absolutely a thing in IT systems: https://www.telcion.com/blog/security-vendors-is-it-better-to-have-one-or-multiple