Imagine how complicated issuing a Motu Proprio expanding use of the "Tridentine" Mass must be? There are many "logistical" issues to work through, canon law to harmonize, rubrics to consider, internal discipline of the Church to maintain. The celebration of the older form must also be "protected" from abuse. Consider this question by a commentor on another entry in this blog, "Scott":
Here’s my question: what is the risk that the Traditional Latin Mass could fall victim to some of the same “innovations” and liturgical abuses we’ve witnessed in the Pauline rite?
Briefly, use of the older Mass could be compromised by priests (and lay people) who really don’t know it very well and, in their good will and zeal, attempt things they don’t understand too well and fill in the blanks with things from the Novus Ordo. So, real instruction/education is necessary.
Similarly, there are problem for the older form of Mass when people celebrated not the 1962 rubrics, but rather rubrics from some earlier edition. Some older priests remember the old Mass well-enough, but they might not have had time to absorb well the changes in the 1962 edition. So, you see far and wide even now in the places where the "Tridentine" Mass is approved for use, celebrations including things that were once part of Mass but really were not long part of the 1962 edition.o
Secondly, there it is possible that the older form of Mass could be abused by making it too rigid. For example, the "Tridentine" Missal didn’t describe HOW people were to receive Holy Communion. That was covered elsewhere. Today there is legislation permitting people to receive Communion in the hand (horribile dictu). Like it or not, that is the law. That should apply as well to celebrations of the older Mass. Most people who would go to the old Mass, won’t want to do that, but… there it is.
There are also a couple oddities to consider. In light of the question from "Scott" a kind of nightmare jumped into my mind. You might know that it was the practice in the "old days" to vest a layman or young cleric, such as a seminarian, to fill the role of a subdeacon in High Masses. The legislation of Paul VI, Ministeria quaedam, got rid of the minor orders and said that the roles of subdeacon were assumed by the acolyte and lector. So, if because of the interpretation of the 1983 Code of Canon Law that says women can "substitute" for acolytes, could you envision dressing a woman in tunic, maniple, biretta and all that get up so that she could substitute for an absent subdeacon? Sorry. I told you it was a nightmare.
Canonists need to work out a lot of things, I think. We will need some patience and common sense.