words - Why 1,000 generations of ungiven Torah, not X years?

The Mainz Anonymous reads, in part (in my own translation):

He created the Torah 974 generations before the world was created, then twenty-six generations [passed] from the creation of the world until Moshe, the leader of the prophets, through whom the Torah was given.

(I'm quoting from the MA in light of this week's topic challenge. In fact, though, there are older sources for this idea, though I'm not sure where.)

Obviously, the midrash paraphrased here is not to be taken literally: specifically, without anyone's having children, there are no generations; and perhaps time did not even elapse (not being created) before the creation of the world. (On the other hand, perhaps it did.) In any event, my question is why the midrash uses generations instead of a more conventional unit of time (such as years).

An initial idea toward a possible answer: Note that Moshe had contemporaries, even Jewish ones, who were more or fewer than twenty-six male-line generations from the creation of the world. For example, Nachshon was twenty-eight generations. So "twenty-six generations" makes sense only if one refers specifically to Moshe, rather then generally to his time. Perhaps, then, generations are not being used as a unit of time at all but as a unit of succession or something like that.

Answer

The source of this midrash is actually the Talmud in Chagigah 13b -- 14a. The Talmud states:

תניא אמר רבי שמעון החסיד אלו תשע מאות ושבעים וארבע דורות שקומטו להיבראות קודם שנברא העולם ולא נבראו עמד הקב"ה ושתלן בכל דור ודור והן הן עזי פנים שבדור

which Soncino translates as:

It is taught: R. Simeon the Pious said: These are the nine hundred and seventy four generations who pressed themselves forward to be created before the world was created, but were not created: the Holy One, blessed be He, arose and planted them in every generation, and it is they who are the insolent of each generation.

Rashi there (sv Kodem) explains that the Midrash means that the Torah was designed to be given after 1000 generations (as it says in Tehillim 105:8 דבר צוה לאלף דור : that which he commanded a thousand generations) but, forseeing that they wouldn't survive without Torah, God pulled 974 generations out (leaving the 26 from Adam to Moshe) and sprinkled them out among the future generations. The evil doers that exist today are thus transplants from a time that never happened (whatever that means).

So, in short, the unit of time used is the Generation as an allusion to the verse quoted above from Tehillim 105:8 which mentions 1000 generations in the context of the commanding of the Torah.