I certainly agree that it would be more convenient to have covariant conversions. The proposal doesn't include them because — while we believe that Result will still be useful after we've added async / await — we don't believe it will still be useful enough to justify the complexity cost of adding those conversions. That is, it will no longer be important for working with Result to be especially convenient.
If you believe that Result should not be added without such conversions, you should recommend rejecting this proposal.