Good question and the answer is I don't know. Some of the earlier brain injuries were iirc due to the hit in the scrum - multiple (as in thousands) of low impact jarring injuries to the brains of players (Steve Thompson for one
) . The rest are surely due to bad tackles (possible in sevens) or the insanity at the ruck (far less so)
It seems rugby is not the only one - NFL's settlement of a class action lawsuit that ran into the billions is one example, as well as, apparently, league and aussie rules.
Agree that player weight has an enormous bearing - these players are fucking huge - and that this could be addressed by changing the subs rule, but rugby needs to move away from the attritional, contact-seeking mess it's got itself into and that requires reform.
All I know is that my albeit dwindling pleasure in the game is not worth one destroyed life like Ryan Jones', let alone hundreds or possibly thousands. It's a disturbing fact that parents are better advised on safety grounds to encourage their kids into boxing rather than modern rugby and that pretty much says all we need to know about the current situation.