I agree with you that we do need to do better about writing more robust software, and type safe languages are a good way to do that.

But what you're saying is as if you suggest that since the metric system is more consistent and more widely used than the English, I as a bolt distributor should start selling my bolts in metric sizes, despite the fact that the nuts that everyone has are in English sizes.

The browser vendors, at least, are working on implement their browsers in more type-safe languages (https://github.com/servo/servo), but even still they have to work with the content that is produced by thousands of different languages, frameworks, and tools, and millions of hand written HTML files, templates, and the like. Just turning on strict XML parsing doesn't make that go away, it just makes your browser fail on most websites.