Precisely. If materialism is true, the noble eightfold path is really redundant. It is only in the context of rebirth that early Buddhism makes sense.
There is no real Tathāgata, in the sense of something you can pin down as ever-present. This does not make the Tathāgata irrelevant as a phenomenon of nature. The Tathāgata is the manifestation of five khandhas, five constituents of existence, imbued with exceptional wisdom. We should be extremely grateful that the five khandhas can manifest in this way, for they are the foundation upon which we may reduce and even eliminate suffering in our lives.
There is nothing mysterious about the Tathāgata or the arahant, apart from their supreme insight into the nature of existence.
I would suggest the following translations are more accurate:
Abrahmacariya: “Not abstaining from sexuality.” Kāmesumicchācāra: “Wrong sensual conduct,” which is really a reference to sexuality.
I believe we tend to read too much into such passages. The Buddha is here speaking to Brahmins, all of whom would have taken the existence of an attā for granted. The Buddha’s idea that someone might just cease at death is revolutionary and different from all philosophies that existed at the time. The simplest and most straightforward way of understanding “there is no measuring” of the arahant who has died and “all ways of speaking [about it/him] are also removed” is that the arahant has just ceased. There is no need to bring in any mystical dimension.
I would argue that the Buddha used such indirect language because he was dealing with a very delicate subject. He would be mistaken for an annihilationist unless he chose his words with great care. To state outright the arahant ceases at death would probably have been misunderstood, and thus the more roundabout and cumbersome formulation.
It is not depressing! It is the highest happiness. It only appears depressing because of the sense of self. It is delusion that is letting you down.
There is an important distinction between the materialist point of view and the Buddhist view, as pointed out by @dxm_dxm above. Materialist have a sense of self, and it is for this reason that death does not appeal to many materialists (although it may of course appeal to those who have had enough of all this). Whether this sense of self is solidified into a view of self or not is really irrelevant. The arahant, by contrast, has no sense of self. For them all there is is suffering; experience itself is seen as suffering. When experience comes to an end, they attain the highest happiness. And they have nothing to lose, since there never was anything apart from changeable and unsatisfactory phenomena. When it comes to an end … hurray! All you have lost is suffering.
This seems overly optimistic to me. The world is, and has been, full of dodgy but charismatic spiritual leaders with heaps of followers. Check out this one, for example.