I agree that some laws can't currently be followed, but that doesn't mean that we shouldn't follow the laws that we can currently follow. "To fulfill the law" means "to cause God's will (as made known in the law) to be obeyed as it should be (NAS Greek Lexicon: pleroo), so Jesus proceeded to fulfill the law throughout the rest of Matthew 5 by correcting what the people had heard bing said and by teasing how to correctly obey it at it was originally intended. The Torah teaches us how to point to Christ and we should live in a way that points to him by following his example of obedience to it rather than a way that points away from him. For example, if you believe that Jesus is holy, then the way to point to him is by follow his example of obeying God's instructions for how to be holy as He is holy. If you believe that Jesus is compassionate, then you should be compassionate in accordance with the example that he set for us to follow, and so forth. The purpose of being a Christian is to live in a way that points towards Christ. The only way that we should cease to follow one of the 613 laws would be if it ceased to teach us something true about who Christ is, but Christ is eternal, such as we should only cease to follow God's instructions for how to be holy as He is holy if He were to cease to be holy. The point of being a Christian is to be like Christ and all of Torah teaches us how to do that.
I said nothing about believing that Jesus' sacrifice not being enough. The reason why God commanded the Torah was not in order to teach us how to live in a way that shows that Christ is not enough. Jesus said that man shall not live by bread alone but by every word that come from the mouth of God, not that we should only live by the Ten Commandments. In James 2:1-11, it criticizes them for committing the sin of favoritism, which is not one of the Ten Commandments, and there are many other things spoken about in the NT that are not one of the Ten Commandments, such as those things listed in 1 Corinthians 6:9-11, Romans 1:29-31, and Galatians 5:19-21, as well as the greatest two commandments. Moreover, the reason why the commandments to love God and our neighbor are the greatest two commandments is that they are inclusive of all of the other commandments, which is why Jesus said in Matthew 22:36-40 that those are the greatest two commandments and that all of the other commandments hang on them, so the position that Jesus taught us to obey the greatest two commandment is also the position that Jesus taught us to obey the rest of the commandments that hang on them.