It always used to be the rule that the CAA did not allow swopping between types - ok between "variants" subject to specified conditions.
I don't think that the comparison with general aviation types or what the military do is valid - we are surely talking about public transport here.
Whilst there are similarities between Boeing aircraft there are some fundamental differences between the 757 and the 727 - how about the number of engines for a start!!
The 727 is three crew and 757 is two crew. That's before we mention EFIS versus round dial.
I am not casting any aspersions on this particular pilot's ability but swopping between types is bound to amplify any potential for error which is, argueably, not acceptable for public transport operations.
Sounds like another case of "dumbing down" to me.
------------------