You are right about the current possibility to get 48+ minutes during a walkover. As for injuries, I'm pretty it has always been the case (or at least for a very long time) that you could sustain an injury. As explained above, if a player can get GS and training minutes during a walkover, it's normal he is at risk to get injured. Unfortunate, but within the game.
At the risk of repeating a conversation that can only go nowhere, as speaking to you is like speaking to a brick wall, its a stupid feature and stupid features only need to be a feature till they're not a feature. The answer "this is how it is, so this is how it is" is incredibly irritating, especially as everything is as it is; until it isn't.
Fouls happen at an abnormal rate during a supposed walkover (by definition a game that does not take place) and since injuries only happen when there's a foul, the team that actually turns up for the game is punished by the increased chance of injury.
As an extreme example, if I was playing a team on tuesday in the league, and then on thursday in the cup, it would be mathematically correct for me to give a walkover in the league game, increasing his chances of an injury occuring, and increasing my chances of winning in the cup. Still, "this is how it is."