First off, let me say that I have never participated in a league using contracts. All good academics know that stating bias up-front allows readers to judge the veracity of remarks
That stated, here is my opinion on the situation....
Of course no one will agree on the value of players with single-year contacts versus multi-year. As mentioned in your post, the contenders have separated themselves from the pretenders, and now seek the players to put themselves over the top. By seeking the prize this year, however, they risk consistent success in succeeding years. In my opinion, this risk should rest with each individual owner. If one is willing to risk the next four years to assemble a superteam, so be it. Do the 1997 Marlins sound familiar?
I understand that other teams would perhaps have trouble with this concept because it could put some out of contention. But those teams unwilling to risk the future rarely deserve to win top honors. Sometimes glory in the present requires sacrificing the future.
From the perspective of the teams out of contention, trading for players with multiple years remaining allows for a stronger chance at contention the following year. Such forsight should not be punished by insisting on competitive balance throughout the year.
My advice: Allow the owners latitude in their trades. After all, some owners will play for this year, and others will take a longer-term approach. Each one of us will have different opinions as to the "correct" approach.