One of the reasons for the concelebration becoming normative is to reduce the number of masses going on simultaneously in a given edifice. Some older churches have 2 or even more side altars, in addition to the high altar and possibly the Table of Repose for the tabernacle.
It was not uncommon to have the main mass (usually missa cantata) going on on the high altar, and separate missa privata at one or more side altars. How much better to have all say the liturgy together, and focus all on the same sacrifice of the Mass? Not that it was terribly much an issue in the US… save for monastic communities.
Now, there have been a few occasions in my life I was glad the Missa Privata was not totally banned… but in general, it should be the exception, while for hundreds of years, it was highly prevalent.
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And as for more clerics reducing extraordinary ministers: technically, lay readers and altar servers are also technicaly extraordinary ministers. The proper minister of the server is the Instituted Acolyte, and the proper minister of the lection is the Instituted Lector. Now, it is permitted and normative to allow these lay readers and altar servers to function without having to devlove the duties upon the celebrant (unlike the Deacon and the Gospel).
But they are still not the ones for whom it is the ordinary ministry.