alt Hacker NewsAdd integration between all the parts to it and you will see why those big companies stay successful.
Not only is managing 20 vendors a nightmare, they all live in their own bubble and moving data from one to the other is normally not that easy.
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No organisation of any size buys everything from one vendor though. Microsoft dominates desktops, but Apple and Google dominate mobile devices, an organisation might have Oracle databases running on Linux servers on top of that, some SaaS suppliers, some desktop software suppliers....
Microsoft has a terrible history of integration even among it's own products and has forced obsoletion throughout. If it's literally you only have a single vendor to pay then you must look for a nationalised solution otherwise you'll just be creating oligarchy.
You can't on the one hand maintain the myth that there will somehow be private competition but then on the other set the barriers so high that only the largest most entrenched monopolies can succeed.
Using standards typically makes a big difference. And having redundancy, so that lack of interoperability/lock in is actually not something you find out after it is too late.