You accurately laid out the two contesting arguments going on about Sage. ( absent the far less salient arguments being made about 'team spirit' and 'leadership' and 'being a quiet team' that frankly don't matter if the system is working from front-to-back ) You are right in saying if we cannot fix the defensive lapses then it is inevitable that either Sage must go or his system must change.
But I still can't see why a five-at-the-back system means we suddenly cannot defend a cross or a set piece. Yes, we are trying to move through the thirds faster so we are taking positional and passing risks that Glasner never had us taking. That has led to turnovers or lapses like the first goal against Forest. If they were the only type of goals we were conceding then a conversation at board level needs to happen about about major changes.
But most of our conceded goals are not happening because the system is being poorly executed and leaving us exposed and conceding goals. Slow build-up oppositional attacks, simple set pieces and corners are leading to crosses into our box that we are failing to clear or even failing to mark against. That is a problem with basic defending! Fail to fix that and no system or team can be successful.
I defend the system because it is getting goals out of JSL, Nketiah, Kamada, Pino and others, making us vastly less reliable on our two wayward attacking stars in Mateta and Sarr, who we have barely missed in terms of goalscoring stats. The new system makes us a far more fluid and less predictable attacking unit and harder to counter. Yes, we cede possession a lot but we did that under Glasner and his regimented system that was easier to defend against. The difference was we did basic defending better under Glasner, maybe because we had a settled back line of excellent CBs.
I think we need Lerma at the back, playing with Richards and Canvot -a settled back line. If Tomiyasu is ever able to be consistently fit or Disasi can prove he can be relied upon, then we can tinker but we need a back three who play together as often as possible and we need to maximise defending crosses in training, assuming we aren't already. And we need to pay whatever it takes to secure a fast CB who can also dominate in the air. If it costs £50M, so be it.
Fix the defence and this team and system can go places. Best of all, it doesn't require exceptional talents like an Eze, Olise or Zaha to make it work. It only needs accurate passing and intelligent, willing runners. I think it's how a club like us, unable to pay enough to get the next Cherki, Tzolis or Dembele, can still keep going forward.