The standard of riders in the PL has dropped year on year for many years. Generally, the average of riders returning after being absent for a number of years should be increased, not reduced because their average was attained when the standard was higher.
However, as I have said many times, the most sensible way is to give them an average based on their average in whichever foreign league(s) they have most recently ridden. It would be easy to devise a formula to convert averages for each foreign league to an assessed PL average. That would, overnight, do away with those who come here when relatively inexperienced, don't ride here for a few years, improve significantly and then return on their original low average and cater for those who have not ridden here for a few years but are on a downward spiral in the twilight of their careers and realistically, wouldn't achieve their past PL average.