You'll have a much harder time breaking into the market with things like a new OS because you have so much more ground to cover today than you would had you started back in the 90s. It's not enough to have a half-decent GUI and a web browser: you'll never get any corporate customers without a significant amount of security hardening, and you'll have to have an extremely compelling reason for them to switch even their servers over (which are run by IT people, who are much easier to train on new technologies than the rest of the workforce), much less their desktop fleets.

Consider how many businesses are built around Excel and either cannot re-create their workflows in LibreOffice Calc/Google Sheets or don't see the value proposition in doing so. You can argue till you're blue in the face that business critical processes shouldn't rely on Excel, and you'd be right, but good luck convincing the people who matter most that they need to change something that still works well enough (especially if they're not privy to the behind-the-scenes work required to integrate other systems with Excel). It's not like Excel is the only thing keeping them hooked on Office.

When it comes to other, less ambitious projects, like email software (not even hosting, just an email client; hosting is its own can of worms), you're competing against companies that either give it away for free or include it in a bundle of other applications businesses really want, like O365. I pay for Shortwave because I loved Inbox by Google that much, but it's a very niche product. I doubt the average person loved Inbox enough to trust a third party with their emails so they can get that experience back, much less pay said third party when the Gmail web interface is good enough for most tasks.

There's a reason we can describe companies like MS as being entrenched: there are no real competitors left. Enshittification works because people aren't inclined to switch without significant upsides. If your selling point is not being actively hostile to your users, why should I believe you will resist doing the same when you IPO or get bought out?