The Agama/Nikaya teachings can be taken under two categories: early teachings (historically) and fundamental/basic teachings (doctrinally).
This is a recent development. Until a few decades ago, outside of Theravada, all Buddhists would have considered the Lotus Sūtra just as historical and fundamental as anything in the Āgamas. Everyone believed the Lotus Sutra had been taught in the same manner as anything in the Āgamas. Of course nowadays there is a distinction between "Early Buddhism" and later developments such as Mahāyāna.
On them were built the Abhidharma, Madhyamaka, Yogacara, Tathagatagarbha and Vajrayana teachings.
Well, this is the present day historian's point of view. However, some scriptures state that the Mahāyāna teachings are not a new development. Take for example the opening remarks in the Brahma Net Sūtra.
《梵網經》卷2：「爾時盧舍那佛。為此大眾。略開百千恒河沙不可說法門中心地。如毛頭許。是過去一切佛已說。未來佛當說。現在佛今說。三世菩薩已學當學今學。我已百劫修行是心地。號吾為盧舍那。汝諸佛轉我所說。與一切眾生開心地道。」(CBETA, T24, no. 1484, p. 1003, b10-15)
At the time, Vairocana Buddha for the great assembly began to speak generally of the mind ground within the inexpressible dharma gates as numerous as sands in a hundred-thousand Ganges Rivers. It was still only like the tip of a hair.
"This was taught by all Buddhas of the past. It will be taught by Buddhas of the future. It is now taught by Buddhas of the present. Bodhisattvas of the Three Realms have practised it, will practise it and now practise it. I have cultivated this mind ground for a hundred kalpas. I am called Vairocana. You Buddhas must transmit what I teach unto sentient beings and open the path to this mind ground."
From this perspective the Bodhisattva path has always been existent and taught by Buddhas of the past. It is not a development of Āgama ideas.
However, that will probably not satisfy the sceptical scholar who is employing literary analysis and building a totally different image of the development of the Buddhist canon.
Another example to consider is the Mahāyānasūtrālamkārakārikā which while nominally penned by Asanga is said to have actually been composed by Maitreya and transmitted to Asanga. Some Japanese scholars doubt this and suggested a monk named Maitreya might have been Asanga's teacher in the flesh. However, Robert Thurman argues that such a bias against Bodhisattvas actually residing in Tuṣita Heaven and being able to transmit teachings to sagely monks on Earth is not truly objective because it is really just a perspective conditioned by a materialist worldview which has its own set of preconceptions and ideas just as the Buddhist worldview does. Thurman concludes that unless evidence is presented to the contrary, there is nothing wrong with holding to the traditional account that Asanga had a vision of Maitreya and received the text through him.
Again, from that perspective there are no early teachings or later teachings. The teachings of Maitreya presumably had been available in Tuṣita prior to the Śrāvaka teachings being taught by Shakyamuni. Maitreya only revealed his teachings on the Mahāyāna when the time was appropriate.
Now, my question is if there's any validity for stating that one teaching is higher than the other.
Traditionally it is the case that there are higher teachings. The the goal of the Śrāvaka teachings is to become an Arhat. The goal of the Mahāyāna is the Bodhisattva path followed by Buddhahood.
There are two perspectives: the teaching and the practitioner. From the point of the teaching, the more forms and methods it has the better. From the point of the practitioner, the stronger one's obstructions are more skilful means are needed to be applied. Thus the most varied teaching fits the largest number of people, while the simplest teaching is for the smallest number. This makes the later teachings lower doctrinally and higher applicably. What do you think?
The premise of what you're saying requires that the Āgamas came first and then the other teachings, such as the Mahāyāna scriptures, really came later. However, if this is not truly the case then what you are proposing is unimportant. In other words, what if the Mahāyāna is not a development on the Āgama teachings and has actually always existed as I suggest above?