if <!DOCTYPE html> is referring now to HTML5, what will happen when a new standard overrides HTML5 ? How will the current HTML5 pages with <!DOCTYPE html> be interpreted in the future?
The same way they are declared now. The Living Standard evolves but is with the exception of deprecated and obsoletes feature completely backward compatible. There will be no need to change the document type declaration. The doctype simply tells the browser to use the current (latest) recommendations.