Question: Is there a .NET-based CMS that offers a large ecosystem of developers, agencies, and ISVs, and has a strong commitment to backwards compatibility?

For a .NET-based CMS with a big ecosystem and deep commitment to backward compatibility, DNN is a good choice. DNN is a full-featured content management system with tools for web content management, CRM, marketing and social engagement. It's got AI tools for content generation and translation, extensibility, fine-grained user control and multisite management. With a big community and many third-party extensions, it's a good choice for large-scale, secure sites in government, finance and education.

Progress Sitefinity is another good choice for a cloud-native digital experience platform. It supports ASP.NET Core, Blazor, React and headless development, but also offers features like AI-powered experiences, codeless page creation and enterprise integration. It's good for managing complex digital experiences across many channels and geographies, and it can be deployed on-premises or in the cloud. Its security is strong, and its modern technology stack makes it a strong contender in the digital experience market.

If you prefer an open-source option, Payload is worth a look. Payload is a headless CMS and application framework designed to let developers, marketers and product owners work together. It's got AI tools for content generation, visual editing and a variety of field types. It's got features like SSO integration, access control and reusable authentication, so it's good for agencies, marketers and developers who need a flexible and extensible foundation for custom business applications.