Very well said, Ben, absolutely spot on .
The complicated issue is that traditionally domestic US is treated outisde international regulations inside the US and that is very visible in R/T phraseology.
When deregulation started, US pilots flying 98% of their time inside the US were thrown outside their borders and exported the bad habbits. That is fortunately no longer much the case, but bringing and mandating ICAO phraseology inside the US still is an issue today.
Alfanumerics, is not a EU thing, but a European Airline thing. There is no regulation for it, it is voluntary. Was introduced by Air Inter in the 80's to differentiate themselves from Air france after a few incidents and similar call signs inside the same company.
From my past experience the most incidents were with similar call signs inside same company , BA1428 and BA 2428 etc.. for this the random alphanumeric is very good.
For an incident between 2 different airlines , there is no real solution . Even if as ATC you spot one ( e.' BA1428 and DLH 1428 in same frequency at same time , it is very difficult to change , as flight plans are published and repetitive.
ATC cannot durably change a call sign on its own ( only inside a sector ) as call signs are what is the reference ID to ATC systems along the route and used for Route charges collection and overflight diplomatic celarances.
A very old problem, looks simple to fix, but it is not.
Sticking to standard ICAO phraselogy and carefully watch out for your call sign is currently the best solution.