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It makes the most sense for a company to spread their risk amongst as many suppliers as possible if their entire business relies on the performance of those suppliers.
Thinking about it, IT hardware and networking doesn’t ever seem to do this. Maybe that’s because it’s lots of items working together to create a system instead of multiple discrete systems.
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