I search for PE (Premium Plus to use the UA vernacular). Cash prices on UA are generally very reasonable, at least for R fares. I am a PE gal on many airlines and the introduction of Premium Plus on UA was very welcomed by me. E+ does not cut it for me and I had been booking away from UA for long haul flights to those with PE.
As others noted often Premium Plus is less than Economy or just slightly more. Then there are other times where Polaris on a P fare is less expensive than Premium Plus (generally an O fare).
I think Premium Plus is a sweet spot for travellers like me. Many people won't pay the small price differential as they are cost focused whereas I am a value buyer, and as I get older comfort becomes more important.
Currently agree with the FT article. I do suspect UA will start to increase Premium Plus pricing but for now works great for me.