I share OPs sentiment, this is supposed to be the "fantasy mode" where I insert myself into the current MLB landscape, not some "hypothetical" one in which the various GM AIs decide what the team(s) are supposed to look like.
Limiting (can't move people on the 40-man roster) or disabling the "Trade AI" for the "Current" (read: title) RTTS season would go a long way in doing so.