>Second, apparently the template language is Turing-complete. Let's say I write a prime sieve in order to generate a page that lists the first 100 prime numbers

You can't. Using recursion (even primitive) is discouraged, there are even automated safeguards that make it hard (but I don't remember what they are exactly).

But I agree with you in general, I think making a visual editor that deals with templates correctly is not an easy task.