The purpose of God in sending any particular light is significant.
God's purpose in sending the law:
To reveal man's inability to attain righteousness by law through works, and thus realize they need a gracious savior. In other words, the law might lead them to look for someone who says, "Come unto me all who are burdened and I will give you rest," because the law helps them to see that they can't do it alone. The burden is to great!
God's purpose in sending the gospel:
To reveal the narrow path to attain righteousness by grace through faith in Christ who fulfilled the law once and for all. The gospel is the appeal of God sent to all creation to "be reconciled to God." It's, 'whosoever believes will be saved' and it calls all who are weak, lost and in need to come and find rest and healing. The purpose is to call all men to faith and repentance so they may be saved.
Does the purpose of God matter? Does His Word return to him void or does it accomplish the purpose for which He sent it? The purpose of the gospel is to make an appeal, and to enable men to come and find rest. The purpose of the law was NEVER to do this. Does the purpose for which the law and the gospel were sent actually matter?
Conclusion: To PRESUME that because mankind is unable to fulfill the demands of the law in order to attain righteousness (which was NEVER its purpose), then man must be equally unable to respond to God's gracious and powerful gospel appeal to be reconciled (which is its purpose) is completely unfounded Biblically.
THE PURPOSE MATTERS!