I'm not familiar with the .NET ecosystem at all, but in general I think the problem with the "just write everything in the framework's layer" for most companies

I'm not familiar with the .NET ecosystem at all, but in general I think the problem with the "just write everything in the framework's layer" for most companies is your whole application becomes tightly coupled to that framework, and more specifically that version of the framework. That's likely a much bigger problem for monoliths which I would imagine is still the majority of apps, even in 2024. Most developers likely don't stick around long enough to see the pains of doing major upgrades on large codebases. I've been using a layered architecture with great success for a large fintech application. We've even migrated from one web framework to a completely dissimilar web framework trivially as the majority of our code in the business layer needed no changes.