Firstly, I believe from the writings and statements of the Holy Father that he sees there has been a rupture in the liturgical tradition of the Church. The dismantling of the traditional Roman Missal in favor of an inorganic and completely novel liturgy was not a positive development for the life of the Church. Restoring the traditional mass to a more regular status, and having it's celebration open for all who desire to make use of this ancient liturgy will do much to mend this rupture. Therefore a time limit seems not to fit this line of thinking.
Secondly, The Holy Father is promulgating this Motu Proprio as a step towards reconciliation with the Society of St. Pius X. The SSPX made the release of the Tridentine rite mass a precondition to further reconciliation talks, and it seems to me that placing a time limit on this decree could cause the SSPX to come to the conclusion that this freedom for the mass they have called for is only a temporary thing.
Although I may be wrong, the two reasons I give above cause me to distrust this particular rumor, but, we shall soon see.