I don't believe businesses "need" Microsoft. Microsoft is entrenched in corporations for a variety of reasons and none of them because Microsoft is technically any good.

It's a little bit of both.

Microsoft's core product is minimizing operational risk, not the software itself. You can piece together your own stack using best of breed options, but you're going to pay double the price or more, and introduce a ton of friction and risk.

Some businesses (everyone outside of the SV tech echo chamber) "need" Microsoft because its risk mitigation, which is the highest technical feature a business can ask for. Backwards compatibility, EntraID is good, and the compliance/purview stack solves nearly all regulatory headaches OOTB.

OTOH yeah there's a bit of legacy entrenchment, both from Microsoft's monopolistic behavior but also because they were the only ones with an "IT In a box" solution for non-tech companies. Having a cohesive identity, security, and device management ecosystem that can scale to hundreds of thousands of endpoints with a few mouse clicks takes a lot of engineering effort that not many others were doing at the time.