Appreciate the obvious effort that went into the calculations. With all due respect, normalization /per day or other variable costs is somewhat irrelevant if the plan doesn't give the option in practice.
Theoretical options not yet offered can blur the issue a bit. Cricket Pay Go is an example I would choose because it is a real world example what is actually being offered. $1 a day unlimited talk on days used only. Att doesn't have a monthly unlimited prepaid plan and Boost , the example cited with the calculations doesn't have a per diem option.At least,not at this time.
The examples made are interesting and bring up a valid point but , one that somewhat hedges the fact that they don't reflect current real world offerings.
With the ferocity of the current price war, one never knows though. Perhaps in the very near future , some of the offshoots you brought up or ones very much like it may well be offered up . Might make sense in a wild west market like this.
And if so, again, the customer will win. Nothing wrong with that.