As true as your post is for a few iPad Minis that wasn’t intentionally what the iPad Mini was designed to be in the first place. It was first released though ironically when no Air Models existed and was better than the iPad 2 which it was shrunken down from
they should of made the Air one or a few things better than the Mini lineup but they made the Mini 3 better than the original Air and the Air 3 and Mini 5 has the same features and hardware (just the Air has the connector for keyboards which Mini 5 lacks)
The mini 2 had lower color gamut display compared to the Air 1.
The mini 3 was released at the same time as Air 2 so the mini was actually lagging behind at that point.
Apart from higher pixel density, I don't think there was ever a time when the mini was better than the current $500-600 iPad.
2012 mini 1 - iPad 4 (A5/512MB vs A6X/1GB)
2013 mini 2 - Air 1 (both A7/1GB, worse display on mini 2)
2014 mini 3 - Air 2 (basically mini 2 with Touch ID vs A8X/2GB RAM)
2015 mini 4 - Air 2 (A8/2GB vs A8X/2GB, both at $399, new 9.7" was delayed to next year)
2019 mini 5 - Air 3 (both A12/3GB, no SmartConnector on mini5)