The Gigacorps have done this, some of them. I have more interest in the ARM argument than in the "custom chip" argument, unless Amazon and co. decide to retail their processors.
Consoles, most OEMs, and all non-gigacorps will be buying off-the-shelf processors.
I'm pretty sure cloud and enterprise sales make up the bulk of their revenue and growth. Not consumer.
That being said there's likely still a few years of runway left for their growth. Even if cloud vendors don't push their own chips aggressively, they can use it as leverage to cut into AMD margins.
I don't see how ARM is an argument. AMD will easily be able to create competitive arm processors, there just wasn't a strong financial incentive to do it before
Consoles, most OEMs, and all non-gigacorps will be buying off-the-shelf processors.