This is a serious question about our understanding of the development of the current Halachah.
When I started my Teshuvah some 30 years ago, I was told that the Sages knew the Whole Torah and their disputes represent different approaches. After studying the Gemmorah every day for 20 years I arrive at a different conclusion - there was no הלכה פסוקה that obligated all the Jews at least from the times of Hillel and Shammai, but the opposite is true - every Rabbi had his own community that he managed Halachicly with little dependence on others.
The sources are too numerous to bring, in general, all the Machlokot show that there was no clear Halachah before that Machloket and in most cases ever after for long long time (until RiFF or Rambam's conclusion etc).
The Machlokot in Rosh Hashanah about Kiddush Hachodesh, for example, clearly show that such a serious Halachah that all the festivals depended on, was not unanimously accepted for generations of Tanaim and Amoraim.
Therefore I conclude, that despite the fact that thousands of "single-phrased" Mishnayot existed in the times of the Tanayim, their interpretations varied so much from Beis Midrash to Beis Midrash that there was no הלכה פסוקה in almost all areas.
Are there Halachic sources that conform to this conclusion?
NB: As @Alex noted, I can not overlook the fact that there were some הלכות פסוקות as listed in Brochos, but still the vast majority of the Talmud is dedicated to the disputes and not such הלכות.