Samsung is partnering with AMD for the GPUs in their Exynos chips. Apple could do that or partner with Imagination again. For Mac Pros they could still stick with dedicated AMD cards. ARM does support PCIe and there are ARM servers that use AMD/Nvidia for GPGPU[2]. So Apple does have options and we'll see what they end up doing.

[1] https://www.tomshardware.com/news/amd-rdna-exynos-samsung-so...

[2] https://www.youtube.com/watch?v=iaY3Ffor-8c