They're ditching Intel and the discrete GPU chips that it needs.

This is the reason Metal isn't introduced on Mac. They're waiting. Waiting until their Macs run on ARM.

I'm not saying it'll happen tomorrow. It may take a few years.

I don't necessarily agree. I think a big part of Metal is to address the limited resource space (such as RAM) that game makers currently face. If you have an iPhone 5S, you're usually fine -- but iOS 7 (and I'm sure iOS 8) takes up a lot more overhead than previous versions. In addition to paving the way for better experiences and better manipulation of the GPU, I also think this about providing game makers a way to better take advantage of the full iOS hardware stack.

That isn't to say there might not be another reason for them to forgo the Mac (whether it's waiting for ARM support or otherwise) -- or that Apple probably isn't actively looking at if/when it can bring an A-whatever to power a Mac, but that still seems a long way off.

I don´t understand.

Is Apple returning to make its own chips trend?

Apple never had their own desktop chips. PowerPC was an IBM architecture. They had created ARM to make chips for Newton, but that's it.

The Mac is not a gaming platform, while iOS certainly is.