You don't seem to understand that if a broke USA can't pay its debts, its biggest problems aren't getting invaded militarily but economic. Even if it defaults on foreign debt, a) new debt is going to be insanely expensive which is bad in the long-term; b) to avoid defaulting on domestic debt (and likely destroying the domestic economy in doing so), taxes would have to go through the roof... which would also be like taking a wrecking ball to economic growth.

Interest on the debt in 2019 is equal to about 60% of the much maligned defense budget, to put it in different terms.

Re: return on debt, that's really not a strong argument when you consider where the budget actually goes. About 75% of the 2019 budget goes to entitlements (of which the majority goes to retired persons who receive more in benefits than they ever paid in) and military, neither of which really seems like they're going to bring in significant returns to justify the debt.

Sure, there are probably some programs where this argument makes sense but my impression is that most large-spending proposals from progressives are more motivated by lofty idealism designed to motivate the voting base than policies where the cost of debt vs ROI is being carefully considered.

That said, irresponsible spending on Washington is definitely a bipartisan sport these days. And as an incumbent politician, why not? You reap the electoral benefits of lavish spending on the voting public, and neither you nor voters in general care much to think any longer-term than the next election cycle.