This is nice, but in no way game-changing. It's true that a lot of XSS slips by because people forget about attribute quoting rules, and (more rarely) expand content directly into inline JS. But it's also true that most XSS still occurs in straight-up content, which is a setting for which every mainstream framework already has effective filtering.
The problem with most auto-escaping templates is that they may save typing (by turning on escaping by default, rather than making developers do it explicitly), they don't save thinking. You still need to inspect each template and make sure that the correct escaping is used for each interpolated string. Context-aware systems like this one (or others that operate directly on structured data rather than strings) allow you to rely to a much greater extent on the computer for that work.
