Sure, if an airline pilot hasn't done any GA flying in 30 years then they'd probably want and need some transition training before going over to RA. But that's a process they've been used to their whole career.
These days, though, a pilot leaving GA for RA might be going, say, from a draggy old Maule to a slippery Pipistrel. (Or a thirty-thousand hour ag pilot going over to ... well, anything). Not exactly what the regs had in mind. So, yeah, while transition training is always advisable, it no longer makes sense to mandate a fixed number of hours for candidates who don't need it. Competency based testing is now fairly well accepted in aviation.
P.S: An interesting FAA take on transition training:
https://www.youtube.com/watch?v=TpxJIr4dsF8
"The lack of transition training has been cited as a causal factor in many GA accidents. Accidents frequently result from pilots being unprepared for challenges presented by the new, or different, aircraft they are flying. Even when pilots are legally certificated to operate aircraft within a specific category and class, significant differences can exist among different types of aircraft within that category and class — thus necessitating the need for effective transition training."