Let me restate more bluntly what my opinion is.
The Scala require construction is useless as specified and implemented because it imposes an unacceptable run-time burden. Language features should be in the language because programmers should use them, not so programmers should avoid them.
The system should be enhanced to make require usable and thus encourage its use. Such an enhancement should provide a mechanism to elide run-time checks of such required assertions in production code while maintaining their use and usefulness in development code. Such an enhancement should not in any way effect the semantics or behaviour of assert.