If you're running a small SaaS company, it doesn't matter all that much what 80% of the world is doing.
I used to feel the same way you do. My apps were all Ruby or Java, usually with Postgres, deployed on Linux.
My past couple of employers, though, have used C# and SQL Server running on Windows Server. And they've both been able to handle heavy workloads without using a huge amount of resources.
I'd think twice about using SQL Server if I were launching a small SaaS app due to licensing costs. But the extra cost of a Windows VM wouldn't hurt much if I had more than a couple of users. And I know from personal observation that a relatively boring Windows/IIS/.NET stack would handle the workload of my SaaS app without breaking a sweat. And there's no reason you couldn't mix in React on the front end. I haven't run into any noticeable GC issues even while running heavy workloads, but I could see it being an issue depending on the type of work you do.
That's not to say you shouldn't deploy a cutting edge web stack on Linux or the BSD of your choice. You can certainly be successful that way too. Perhaps a balance is best: use boring, battle tested technology where you can, and use the cutting edge tech in places where it will give you a competitive advantage.