However, I have always got the higher Avios due to a J fare rather than an I fare.

The fare has nothing to do with this. You earn Avios according to the booking class in to which that sector has been booked. It is not to do with whether you have bought a J class fare or an I class fare. If the sector is booked in J class, you earn for a sector flown in J class.

The point I wanted to make was that the rationale for higher Avios on a J fare is likely based on it being a more expensive flexible fare which in this case it is not.

That's true. But like so many things with earning frequent flyer points, the systems have to be much simpler than that.

Otherwise, airlines would I'm sure have already found ways of penalising frequent misusers of ex-EU tickets!

My impression is that "fully flexible" fares are so expensive that it would be cheaper to just book 2 or 3 flights in a sale and abandon some of them, or wait until your plans are 95%/99%/99.99% confirmed even if the prices for a non-flexible fare are higher by then.

It may be a different story when your company has a corporate deal / discount and you need to make 5-10 date changes close to the date of departure.

Back in the 90s our corporate travel department always recommended to buy full fare tickets so we could easily change flights. You could literally show up with your Y or F paper ticket for a UA flight and check in for an AA flight for example. We still often booked discounted fares since we had some budget responsibility. Also - we were required to fly coach under 5 hours unless the flight was a red-eye. We booked lots of full fare red-eyes ;-)