I encourage you to look close at those “great lengths.” I don’t think they say what you think they say.
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“Do not think that I have come to abolish the law or the prophets; I have come not to abolish but to fulfil.” The way the statement starts leads us to expect something like “not to abolish but uphold.” But Jesus ends with a surprise: he has come to fulfil the law and prophets. To fulfil something, you take precedence over it. In this claim, Jesus both denies that he’s doing away with the Scriptures and relativizes those Scriptures beneath himself.
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“For truly I tell you, until heaven and earth pass away, not one letter, not one stroke of a letter, will pass from the law until all is accomplished.” Jesus embeds standard words of Jewish piety between two “until” clauses that subvert the whole sentence – especially since the law being accomplished sounds sneakily similar to the law being fulfilled. Jesus moves the focus from the perfect preservation of the law to its eventual eclipse. By contrast, Jesus later declares that “heaven and earth will pass away, but my words will not pass away.” It’s almost like a schoolyard argument where “for a bazillion years” gets countered with “to infinity plus one!”
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“Therefore, whoever breaks one of the least of these commandments, and teaches others to do the same, will be called least in the kingdom of heaven; but whoever does them and teaches them will be called great in the kingdom of heaven. For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.” Disobey one of the laws, and you’re least in the kingdom. But if you don’t surpass those most religious about following the law, you won’t get in at all. As the end of his sermon demonstrates, obeying Jesus – not the law – is what determines who’s in and who’s out. And then, later on, Jesus turns around and says even the least in the kingdom is greater than John the Baptist! Least and great are chimeric categories in Matthew.
In these verses, Jesus provides his justification for giving a word that goes beyond the law. But to read these introductory words as propping up the law or claiming it will last forever or be the litmus test of the kingdom is to ignore both the words themselves and the shocking teaching that follows. That teaching probably necessitated the defense. Jesus probably wouldn’t have said “I have not come to abolish the law” if his radical approach to the law hadn’t led some pious Jews to conclude that a law-abolisher was exactly what he was.