Emes v'yatziv

The gemara says in Brachos on daf 12a that if you don't say emes v'yatziv then you have not fulfilled your obligation. Which obligation?
1. The Tur learns that this means the Mitzvah of Krias Shma k'hilchasa. Obviously you've fulfilled the mitzva of Shma but not to the exact way that Rabonon said you should.
2. The Hagahos Maimonis (k"s 1:7) says that you haven't fulfilled the obligation of birchas krias shma. Of course, you've fulfilled all your obligations d'oraysa but there is the chiyuv d'rabanon of brachos that you haven't completed.
3. I thought the simple explanation of the gemara was saying that there is a new chiyuv from Tehillim that we need to say the praises of Hashem and that is the obligation that is not fulfilled. According to the other pshatim, the gemara never explains why this is a requirement of krias shma or birchas krias shma. It's just an obligation but who says that it should impact your fulfillment of the other mitzvos?