Why should one study both the prescriptive part of the Veda and the Upaniṣad?

In his Seśvaramīmāṃsā, Vedānta Deśika (XIII c., possibly the most influential Viśiṣṭādvaitin after Yamuna and Rāmānuja) describes the link between the necessity of studying the Veda from an epistemological perspective and the necessity of including in it also the Upaniṣads. I can fully understand his first argument (also the Upaniṣad are valid, because their intrinsic validity is not invalidated due to faults found in their speaker), but I am puzzled by the next one. It seems to say that in the Brahmasūtra and its commentary the validity of the Upaniṣad will be established, whereas in the Mīmāṃsāsūtra the Upaniṣad are acknowledged as an important part of the Veda, but their [independent] epistemological validity is not admitted.

Everything can be a proper (samañjasa) object, once an instrument of knowledge has cleared it up. Hence, a complete reflection on the way instruments of knowledge work is of significance || 20 ||

“A reflection on the instrument to know dharma”: so has [the subject] been in general denoted.

Hence, also a reflection on mantras, commendatory statements, smṛti etc., will find its place in it || 21 ||

In the reflection on the means for knowing the nature (rūpa) of prohibitions and injunctions,

also a reflection on the means for knowing the adharma which is prohibited will be present [in his commentary on MS 1.1.2 Vedānta Deśika elaborates on the subject of adharma being included in the investigation on dharma] || 22 ||

[Obj:] But in regard to the [Upaniṣadic] sentences about the brahman, who rely (viśram-) on the own nature [of brahman], there is no status of being a means for knowing the dharma; [hence] there should not be any reflection on them here || 23 ||

[Reply:] Let it not be like that! Since here one undertakes an inquiry (jijñāsā) on the meaning of the Veda in general, the validity of the whole Veda has to be described from the beginning. || 24 ||

Hence, since an instrument for knowing dharma is an instrument of knowledge because it has no faults on the part of the speaker, also a sentence on brahman is established to be a means of knowledge in general [for both dharma and brahman, and not just for brahman] || 25 ||

The reliance (viśrānti) on the own nature in general will be then discussed in the Śarīraka (Brahmāsūtra),

in fact, here, although there is no mention of the [Upaniṣad statements], the validity [of the whole Veda] will be established

so much that, insofar as they do not aim at their own nature, and are [rather] to be supplemented to a prescription,

the reflection [on the Upaniṣadic statements] is then reached, but not an instrument for [their] validity || 27 ||

(Seśvaramīmāṃsā concluding verses of the commentary on MS 1.1.3) (My previous translation has been improved by Marion Rastelli)