We can also question whether schema-less XML and JSON documents (or BLOBs, or images, or free text) belong in SQL Server in the first place. The user community has already asked for MongoDB style schema validation in Azure CosmosDB, and it seems the product team has indicated it's on the roadmap for 2019.
https://feedback.azure.com/forums/263030-azure-cosmos-db/suggestions/33464752-support-for-document-validation-using-json-schema
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho