That's an out-of-thin-air assumption, most likely invalid.

F class is currently used for upgrades as well as discounted first class fares. By using R class for upgrades and keeping F as discounted first class, it would be more reasonable to assume that the new R class inventory would not equal the old F class inventory. If it was the same, then there would be no need for a new booking class.

Hence, R class inventory will most likely not equal the current F class inventory. Whether there will be more R class inventory than current F class inventory is up for speculation.