How can you say that we were nowhere close to hitting diminishing returns? How do you know that tripling stimulus spending wouldn't have given us significant diminishing returns? This appears to be all theory and speculation with no real basis in fact (or historical fact as you indicated). I see no compelling support that Delong and Krugman were indeed correct.
I don't know at what level you would hit diminishing returns, but logic says it is most likely that diminishing returns would happen gradually as spending increased. Considering that fiscal multipliers were substantially above 1 even with all the stimulus spending going on that strongly indicates we had a ways to go before we dipped below 1.
It is very odd to say you see no argument that they were correct when they predicted the following things: multipliers above 1 (correct), large deficits won't cause inflation (correct), large increases in the monetary base won't cause inflation (correct), that the stimulus would be insufficient to bring us out of the recession quickly (correct). Considering how difficult economic predictions are that's knocking it out of the ballpark, especially when what they were saying was directly opposed by most conservatives.
is your argument that even modest increases in stimulus would quickly push the multiplier below zero? If so, please provide literally any evidence for this. Considering the fact that countries enacted a wide range of amounts of stimulus relative to GDP and no evidence of this emerged, you are going to have a hard time.
You basically seem to be setting up an impossible hurdle in order to avoid admitting Krugman was right, that because no one enacted what he deemed to be sufficient stimulus that there isn't any evidence that he was right. The economics literature says otherwise, as does the paper I linked before. The data speaks for itself.
