Consider what happened after the redaction of the Mishnah. The sages of the time generally accepted that the Oral Torah as such was now 'canonized' in some way and could no longer be disputed. Of course, the Mishnah and attendant literature (Tosefta, Midreshei Halacha, etc.) preserved differences of opinion about various matters but those disagreements were also preserved as part of the Oral Torah.
In any event, rabbis and students continued their learning, presumably in a way similar to the time before the Mishnah was written down, using what we call Talmudic logic. This method of Talmudic thought/logic was preserved in so many words in the Gemara, both in the Talmud Bavli and the Talmud Y'rushalmi. Mainly the Gemara is preserving discussions.
One might have thought that the point of the Gemara, then, was to preserve the methodologies which were employed by the sages of the time to understand the Mishnah; thus, the Gemara would have stood for all time as another way of preserving the Oral Torah--by preserving those kinds of discussions. The Gemara did not seek to necessarily decide halacha in every instance but it did seek to probe and understand the Mishnah as well as it could.
However, by the time the Talmuds were redacted (if that is the proper term-a more complicated issue) it was understood that the discussions of the Gemara were not just exemplary of how discussions of the Oral Torah should go; rather those discussions were viewed as authoritative understandings of the Mishnah. By preserving the Talmud with the Gemara, the Oral Tradition came to another layer of cooling off. From the time of the Talmud, new conversations about much of the Mishnah was stifled in favor of the recorded conversation of the Talmud.
I am not saying there is anything inherently wrong with the discussions of the Talmud--not at all! They are brilliant on many levels and endlessly fascinating. But I am pointing out that once they were written and given the stamp of approval by the later redactors of the Talmud, they had the effect of stifling new and creative discussion, at least to some degree.