They had already plateaued in 2014/2015 with Haswell/Broadwell. They've basically been releasing that same CPU with minor tweaks to power consumption and codec support for 8 years now.
At the time, it was hard to notice, but reviews at the time absolutely noticed the minor CPU update (https://www.theverge.com/2015/4/9/8375735/apple-macbook-pro-..., search "Broadwell"). Another funny aspect of that review: it mentions out 10+ hour battery life for the MBP as a nice, but hardly astonishing spec. 9 hours 45 minutes with Chrome was the worst case. It's amazing to think how bad the 2016-2019 MBPs were in comparison, to the point where getting back to 10 hour battery is an amazing Apple Silicon feature!
At the time, it was hard to notice, but reviews at the time absolutely noticed the minor CPU update (https://www.theverge.com/2015/4/9/8375735/apple-macbook-pro-..., search "Broadwell"). Another funny aspect of that review: it mentions out 10+ hour battery life for the MBP as a nice, but hardly astonishing spec. 9 hours 45 minutes with Chrome was the worst case. It's amazing to think how bad the 2016-2019 MBPs were in comparison, to the point where getting back to 10 hour battery is an amazing Apple Silicon feature!