It's a good point and already on my list, in fact.
The main reason why it can't be implemented yet isn't really because of replays, because with a logic change like this, LFS could in theory be made to work "the old way" for an old replay.  The problem is online compatibility.  It's too small a change to be worth splitting the community.  An online incompatible patch means that all hosts and all players must upgrade and causes quite a lot of confusion.
What that means is that multiplayer incompatibility gets squeezed into the same patches as physics incompatibility, because physics incompatibility also implies multiplayer incompatibility.