I think it simply case of having singular focus on building L4. If something does not bring L4 on market faster, then it gets 0 attention.
Let's face it, us early FSD customers and promises given to us are a distraction. Way how Tesla seems to think is to just accept broken promises as necessary casualties of war for bringing FSD on market at wide scale first. We can bitch about it, we can lose all trust in Tesla, can sue them, ... Tesla just lets their army of lawyers deal with it - whichever way is cheapest on the short term. This way their engineering teams are not distracted.
From engineering point of view, I can respect the focus. From business point of view, I despise their shortsighted lack of understanding the value of customer trust.
Also, I think that including MCU2 upgrades to FSD purchase price would still be cheapest option. I believe that it was actually promised when car was advertised to have all HW needed for FSD and Elon saying that it that would not be the case Tesla would make upgrades for free. MCU2 upgrades would silence early FSD customers for several years and thus reduce the number of cars Tesla would need to eventually somehow deal with if MCU2 is enough.