Was Rashbag Reform? (Or, at least, not Orthodox?)

Here's a mishna from Masechta Kerisus (1:7 - Soncino translation).

If a woman had five doubtful births or five doubtful issues she need bring but one offering, and may then partake of sacrificial flesh, and she is not bound to bring the other [offerings]. If she had five certain issues, or five certain births, she brings one offering and may then partake of sacrificial flesh; but it is still her duty to bring the other offerings.

It once happened in Jerusalem that the price of a pair of doves rose to a golden denar. Said R. Simeon b. Gamaliel, "By this sanctuary, I shall not go to sleep tonight before they cost but a [silver] denar!" Then he entered the Beth Din and and taught; If a woman had five certain births or five certain issues she need bring but one offering, and may then partake of sacrificial flesh, and she is not bound bound to bring the other [offerings]. Thereupon the price of a pair of birds stood at a quarter of a [silver] denar each.

To me, this seems pretty radical. Here we have Rabbi Shimon ben Gamliel, a nasi, changing a halacha purely out of social justice concerns. Now, I don't know a lot about this area of halacha, but I see few possibilities here.

Option A is that the original teaching of the Mishnah, that a woman was obligated to bring additional offerings, was mandated by the Torah. Then, Rashbag, in order to lower the prices of sacrificial doves, changed the meaning of the Torah to remove that obligation, because doves were too expensive. This is the most radical interpretation. This would be like changing the meaning of the constitution, something no Supreme Court justice would ever admit to doing. And certainly not for social justice purposes! Plus, you know, the constitution is man-made, and is thus an incomplete and flawed document. But the Torah is divine and perfect? How can Rashbag just change it to lower the prices of doves in the marketplace?

Option B is that the original teaching of the Mishnah, that a woman was obligated to bring additional offerings, was the consensus Rabbinical interpretation of the verses of the Torah. Meaning, that the Rabbis sincerely thought that a woman was Biblically required to bring additional offerings. Then, Rashbag, in order to lower the prices of sacrificial doves, overruled that interpretation of the Torah to remove that obligation, because doves were too expensive. This is still very radical. This would be more like uprooting a decades-old constitutional doctrine, in favor of social justice. Again, this is super-rare in constitutional jurisprudence. It's stranger, though, because while one Supreme Court may very rarely be comfortable saying that a previous SC's understanding of the Equal Protection Clause was wrong, or outdated, I would think it impossible to read of a Tanna taking this view of a previous Tanna. Were the older generations wrong? Did they misunderstand? But that doesn't even seem to be his argument. He doesn't seem to be saying that the previous generations of scholars misunderstood; he's just changing the rules because the rules are making doves too expensive! Weird!

Option C is that the original teaching of the Mishnah, that a woman was obligated to bring additional offerings, was a Rabbinical statute. Meaning, as far as the Torah is concerned, all a woman need bring is one, but the Rabbis said, for some reason, "Nah, bring five." Comes along Rashbag, and says, we're changing that Rabbinical law, because doves are too expensive. This is the tamest explanation, but still seems pretty interesting, because Rashbag is changing a d'rabbanan just because he feels doves are too expensive. I can see telling someone that they're still yotzei their obligations with one, but actually rewriting the law to make the price of a commodity drop? Still weird. Additionally, he doesn't explain his reasoning. Presumably, the Rabbis had a reason to mandate five. It may be an important reason, it may be not so important. It could be that ensuring a cheap supply of doves is more important. But what was the reason for the original ruling? Also, I'm a complete am ha'aretz in this field, but, facially, it seems strange to me that the Rabbis would mandate additional offerings for no reason. I mean, are they trying to put up a syug l'Torah, "just to be safe"? Safe from what? And do the Rabbis have the authority to mandate sacrifices that are not required by the Torah? Is there such a thing as korbanot mid'rabbanan (Rabbinically created sacrifices)?

In any case, I think it's pretty radical that we have a Tanna over here willing to change a straight up halacha, just to lower the price of doves; for social justice, really. Sounds almost Reform! Now why can't we solve the agunah crisis?