While the Vedic Thought considered religious activity as an end in itself, the Upanishads emphasized Jnana and renunciation of activity.
Since both path of activities were one sided, I believe the Lord had to descend personally to correct their limited outlook of activity which was explained in His Gita.
He has drawn the idea of activity from the Vedas and desirelessness from the Upanishads and Synthesised the two - activity and desirelessness - into desireless activity, which leads to the maintenance of the World.. Bhakti.
There is no doubt about the importance of the Vedas, since from there on, the Lord teaches the rest.
Love you Krishna