I like this question but we must take heed from this kind of thinking. The Prophet (saws) was the living Quran among his duty was to explain the revelation that was being delivered.
Here is an instance found in the hadith.
"I came to the Prophet (ﷺ) while I had a cross of gold around my neck. He said: 'O 'Adi! Remove this idol from yourself!' And I heard him reciting from Surah Bara'ah: They took their rabbis and monks as lords besides Allah (9:31). He said: 'As for them, they did not worship them, but when they made something lawful for them, they considered it lawful, and when they made something unlawful for them, they considered it unlawful.'"
حَدَّثَنَا الْحُسَيْنُ بْنُ يَزِيدَ الْكُوفِيُّ، حَدَّثَنَا عَبْدُ السَّلاَمِ بْنُ حَرْبٍ، عَنْ غُطَيْفِ بْنِ أَعْيَنَ، عَنْ مُصْعَبِ بْنِ سَعْدٍ، عَنْ عَدِيِّ بْنِ حَاتِمٍ، قَالَ أَتَيْتُ النَّبِيَّ صلى الله عليه وسلم وَفِي عُنُقِي صَلِيبٌ مِنْ ذَهَبٍ . فَقَالَ " يَا عَدِيُّ اطْرَحْ عَنْكَ هَذَا الْوَثَنَ " . وَسَمِعْتُهُ يَقْرَأُ فِي سُورَةِ بَرَاءَةََ : ( اتَّخَذُوا أَحْبَارَهُمْ وَرُهْبَانَهُمْ أَرْبَابًا مِنْ دُونِ اللَّهِ ) قَالَ " أَمَا إِنَّهُمْ لَمْ يَكُونُوا يَعْبُدُونَهُمْ وَلَكِنَّهُمْ كَانُوا إِذَا أَحَلُّوا لَهُمْ شَيْئًا اسْتَحَلُّوهُ وَإِذَا حَرَّمُوا عَلَيْهِمْ شَيْئًا حَرَّمُوهُ " . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ غَرِيبٌ لاَ نَعْرِفُهُ إِلاَّ مِنْ حَدِيثِ عَبْدِ السَّلاَمِ بْنِ حَرْبٍ . وَغُطَيْفُ بْنُ أَعْيَنَ لَيْسَ بِمَعْرُوفٍ فِي الْحَدِيثِ .
Grade : Da'if (Darussalam) English reference : Vol. 5, Book 44,
Hadith 3095 Arabic reference : Book 47, Hadith 3378
Aside from explaining the Quran we also have Hadith that mention certain elements that are to take place that are not mention, mention indirectly or mention but doesn't go in all the details.
For instance Ad-Dajjal (Anti-Christ, False Messiah) is not mention directly in the Quran but some verse may indirectly refer to him. This is a well know being that will reveal itself in the end times to deceive mankind (even Christians take it as important).
Once Allah's Messenger (ﷺ) stood amongst the people, glorified and praised Allah as He deserved and then mentioned the Dajjal saying, "l warn you against him (i.e. the Dajjal) and there was no prophet but warned his nation against him. No doubt, Noah warned his nation against him but I tell you about him something of which no prophet told his nation before me. You should know that he is one-eyed, and Allah is not one-eyed."
حَدَّثَنَا عَبْدَانُ، أَخْبَرَنَا عَبْدُ اللَّهِ، عَنْ يُونُسَ، عَنِ الزُّهْرِيِّ، قَالَ سَالِمٌ وَقَالَ ابْنُ عُمَرَ ـ رضى الله عنهما قَامَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي النَّاسِ فَأَثْنَى عَلَى اللَّهِ بِمَا هُوَ أَهْلُهُ، ثُمَّ ذَكَرَ الدَّجَّالَ، فَقَالَ " إِنِّي لأُنْذِرُكُمُوهُ، وَمَا مِنْ نَبِيٍّ إِلاَّ أَنْذَرَهُ قَوْمَهُ، لَقَدْ أَنْذَرَ نُوحٌ قَوْمَهُ، وَلَكِنِّي أَقُولُ لَكُمْ فِيهِ قَوْلاً لَمْ يَقُلْهُ نَبِيٌّ لِقَوْمِهِ، تَعْلَمُونَ أَنَّهُ أَعْوَرُ، وَأَنَّ اللَّهَ لَيْسَ بِأَعْوَرَ ".
Sahih al-Bukhari 3337
Book 60, Hadith 12
These are just a few instances of what we find in hadith's.
So to answer your question: Surely, if you place holy importance on the Hadith, then in essence what you are saying is that the Holy Quran isn't/wasn't as complete as it should be. If it was, you wouldn't need anything else.
I woudn't say we place "Holy Importance" some group within this Ummah might do that. But the important aspect of the Hadith is to explain events that took place in the time of the Prophet (saws), which includes explaining the Quran and living the Quran aswell. Also a hadith must be well authenticated and one must not give more importance to hadith then Quran.
