How does the idea that each unit has an equal chance of being used (which is technically from a probability perspective) require us to the use the gross system?
The problem the second standard (Formally known as the gross system) ran into was that units who gave a mediocre performance from chapter 1 all the way to final come out better than pre-promotes who join around 3/4th of the way in and are amazing for the whole of their existence, because to account for what could have been accomplished had said mediocre growth unit been replaced with someone else would be to invoke the first standard (Known as the net system). Someone would need to be so bad that trying to use them actively worked against you in absolute terms for availability to not be an unconditional benefit, and that is unlikely to be the case for anyone in this game at least.
To be honest I do not see the problem with this.
A second, much less discussed, problem is that if everyone is equally likely to be in play, then no good reason exists to assume that the lowest turn strategies are being used, because those are premised on the best units being in play. This does not result in units being impossible to tier, because you can still put Titania above Soren for having the potential to help achieve a lower turn count, even if it is up in the air whether that particular turn count with be achieved. However, when comparing Soren to, say, Ilyana, you no reason to assume (Or not to assume) that the preceding chapters were a Titania stomp, because the player, despite the apparent assumption that he/she desires clearing in fewer turns than more whenever possible, is under no obligation to give Titania even a single kill for the entirety of the game. This has profound implications for how fast units in Soren's position can be expected to grow, and it creates a huge number of other problems as well. For example, if an optimal team can clear X chapter in 6 turns with optimal tactics, and a sub-optimal team can clear that same chapter in 8 with equally optimal tactics, and both outcomes are considered acceptable by the list, then why should an 8 turn clear with the optimal team, using sub-optimal tactics, not also be considered an acceptable outcome? Attempts have been made to draw a distinction between "Normal Efficiency" and "Maximum Efficiency", but any dividing line that is drawn is sure to be completely arbitrary.