Max,
This is not exactly true the way you wrote it.
SP allows a priest to “privately” say the EF at any time. A Bishop’s permission may be needed to celebrate it publically. It depends on the Bishop.
From what I understand, a priest cannot eliminate an Ordinary Form Mass for an Extraordinary Form mass if the numbers don’t work out.
For example: Lets say the church sits 500 people for Mass. The parish has 5 weekend Masses and 2500 people attend each week.
1 Mass on Saturday evening and 4 Masses on Sunday. The parish has 2 priests, and they are only allowed to celebrate a max of 2 masses a day (in that dioceses) without special permission from the Bishop.
Now, let’s assume that only 100 people want the Extraordinary Form mass.
So they presents a problem, because the only possible time slot to add another public mass (unless the bishop gives permission for a priest to celebrate 3 masses) is to add a second Saturday night mass.
However, adding a 2nd Saturday mass could become a potential problem due to weddings, funerals, and Saturday morning masses.
Also, what happens when the priest who celebrates the Latin Mass is sick, or retreat, etc? Will the parish have coverage? Because a regularly scheduled, public mass really isn’t supposed to be canceled.
However… in that same scenario, if 500 people want the Latin Mass, then it’s not an issue to convert one of the OF masses to the EF (if there is priest coverage).
From what I understand, this is why many priests are in favor of Personal Parishes dedicated to the Extraordinary Form.
I pray this helps.
God Bless