[jerf]

"The power/guarantee trade off isn't a fundamental one, even though in current languages you often have to choose between the two."

It is on the Pareto frontier of the two. Which verges on tautological, but it's still a useful observation. If you can trivially extend the power of a construct while maintaining the same guarantees, the construct was simply broken, and extending the guarantees of a construct without affecting its power means simply that the guarantees were excessively conservative in the first place.

(This can almost be copied and pasted any time anybody claims two properties of a program aren't in conflict, the most common probably being security and ease of programming.)

We may be wrong about where the frontier is... but I doubt we've very wrong.