It's not that Intel's not given the choice -- that would still be very far away. It's that Apple's move would be the first real example of overlap in computing space. Others can follow and eventually claw at Intel's market share in laptop/desktop market.
It would send a signal that Intel isn't the given choice if you've made a vast investment developing your own CPU line over the years.
The rest of the ARM ecosystem is trailing behind Apple, but it is following. If Apple switched, 2-3 years later the rest of the laptop/desktop space would be in the same position.
No, you can already use other vendors chips. What it would say for consumer products is that it isn't the given choice provided you developed and ported your own OS (or -ha- use linux).