Not sure of the source of your 15% but I'm willing to bet that by value it's more - no Celerons in Apple's line up. Plus Apple wouldn't be going down this route if it didn't expect to grow market share - and although people don't care if it's M1 or i5 they do care if the experience is better.
Then Apple's success with the M1 will spur others - I would not be surprised if Microsoft follow them down the same route.
> Apple wouldn't be going down this route if it didn't expect to grow market share
Marketshare is not what Apple is about. Apple is about profitability and control. Their move to own silicon is driven by improvements in the reliability of their build pipeline (no more waiting for tic-tocs and whatnot) and tighter control / integration of their whole stack (same arch on phones and pc). That these chips happen to perform so well that they are potential market-growers, is a welcome coincidence.
Growing marketshare but profitably and without impairing the brand is what Apple is about. That's why we have the iPhone SE. The M series lets them do that with the Mac now. And more Macs implies more Apple services sales.
It's certainly partly defensive - they were frustrated with Intel - but Apple would only make a move of this scale if it thought it created business opportunities for them.
Then Apple's success with the M1 will spur others - I would not be surprised if Microsoft follow them down the same route.