I'm not saying this is definitely wrong but equally there's nothing especially insightful about saying players tend to be less athletically dominant at the end of their careers than the start, and I'm not sure why it would have anything to do with playing rugby union instead of league. Tuqiri left league at 23 and came back at 31. Rogers was 25/31. Thorn was 25/30. Burgess was 28 when he got back to Souths and he started off fine, but his body broke down - hardly surprising for such a physical player and something that I tend to assume would have happened even if he'd played league throughout.
I could also name a dozen physically incredible players who you wouldn't have wanted to sign in their 30s and who never played a game of union. Inglis was done at 31. Shaun Johnson hasn't been the same player physically for at least four years, and he's 32 now. Matt Moylan is 31 and he's had to reinvent his game because he doesn't pose the same threat ball in hand as he used to.
TBF I've often argued on here with people who want to sign blokes who may well be past it - or at least well past their peak - because they don't seem to understand that players age. A player of around 30 is very, very unlikely to be the same physically as they were at 22. I just don't think that has anything to do with playing rugby union.