i don't really see no strings money as a particularly interesting choice or an ethical conumdrum.
It would be an ethical conundrum for the MC because whilst having that kind of money would be great for him in terms of helping his Dad, helping the DIKs, helping Maya, etc, he just made this grandiose speech about not relying on others to pay your way and working hard to earn it for yourself.
So it would be a decision between sticking to his principles about hard work and earning your own living, or being a hypocrite by accepting that money and relying on it to solve all his problems.
Rusty's not cut off, he's just tapped out. I'm sure his dad will give him money again in the future, but it won't be soon enough to give the DIKs a place to live (and party) in the meantime.
Hence the MC's speech. They can salvage what they need in the short term, then slowly rebuild into something that works for them over time. If they get more money fine, but they shouldn't be relying on it.
I call that "cherry picking" where you reach in and grab part of a conversation to support something but ignore everything else that doesn't. We're just going to have to agree to disagree.
No where in there does it say that Rusty's Dad cut off Rusty. Rusty doesn't make some speech about how now he's poor because his dad said eff you no more money. He doesn't say that he won't help in the future. He tells Rusty to use the "buffer money" in the safe. However, he doesn't know Rusty spent that.
I still think the speech by the MC was inspired by Sage's living in a bubble conversation and the MC's upbringing by his Dad who never had a lot of money. So the MC is already trying to be semi-self sufficient. Rusty is a spoiled little rich guy who is used to Daddy saving him. However, the MC wants to grow up and be an adult. That's what his speech is about not OMG NO MORE MONEY
What exactly is there to ignore? Rusty says his Dad told him, "Enough is enough", with regards to helping him out, John Boy asks, "Is this it for us"?, and Rusty says, "I don't have enough money to fix this and
to continue to run the frat the way we've done."
The DIKs are all thinking it's over for them, so if this was just about Rusty's Dad not paying for the repairs but still paying for everything else, why would would they think that? Why does Rusty say that he can't "continue to run the frat the way we've done", if his Dad would still be paying for everything? Answer: because he isn't, and that's the point. Rusty's line is pretty much confirming that the money they have in the safe is not just all the money they have right now, but it's all they money will ever have because his Dad is not going to be funding the DIKs anymore.
This moment directly correlates with the earlier conversation with Sage because it was basically a foreshadowing of what's to come and it's what makes the MC realise and understand what needs to change with the DIKs and gives him the drive to be that agent of change for them.