I don't think they will switch the whole product line to ARM. My guess is that they will have a line of Intel computers for a long time for professional who need them.
Apple already supports devices on different chips (Intel on laptops, ARM on iOS). There is nothing strange about them keeping laptops with different processors, depending on the intended use.
As I said before, the article states expressly that they're moving everything:
> Like it did then, the company plans to eventually transition the entire Mac lineup to its Arm-based processors, including the priciest desktop computers, the people said.