Well my 2013 2.3GHz i7 matches the 2017 i5 iMac in Logic track count. So I know the two machines are about equal in power. That's what the Logic test IS good for.
I also know that I hit the limit of CPU on my last track and had to start bouncing audio to continue.
Many of the latest plugins, like Slate Mix Rack and Soundtoys Effects racks can hold multiple plugins in the one shell.
Every track I run has a Slate mix rack with a bare minimum of a console emulation, a pre amp emulation, an eq, a compressor and a "lift" plugin to get the correct tone. Then each track has at least a Soundtoys effects rack with filter, modulation, stereo spreader and often more.
Each track I run has a minimum of 10 plugins running. Then there's reverb which can be a real CPU killer, audio gates, buss channels with dynamic processes and the master buss.
That's not even looking at instruments!
Granted I make electronic music. And like I said people recording and mixing acoustic instruments may not need such power. But the idea that an i5 will do it all no sweat, with the increasing quality of 3rd party plugins is in my opinion false.
The current i5s would not run what I need them to with any spare overhead on day one. There's no way I'd consider on a good investment.