To me, the question is  "what are the logical conclusions that longtermism leads to?" The idea that as of today we have not exhausted every intervention available is less relevant in considerations of 100s of thousand and millions of years. 

I suspect that if someone had an idea about an intervention that they thought was super great and cost effective for future generations and awful for people alive today, well they would probably post that idea on EA Forum just like anything else, and then people would have a lively debate about it.

I agree. The debate would be whether to follow the moral reasoning of longtermism or not. Something that might be "awful for people alive today" is completely in line with longtermism - it could be the situation. To not support the intervention would constitute a break between theory and practice. 

I think it is important to address the implications of this funny situation sooner rather than later. 

I think you raise a key point about theory of change and observed practice.  

I think we're in a funny situation where maybe there are these tradeoffs in the abstract, but they don't seem to come up in practice.

This "funny situation" means that something is up with the theoretical model.  If the tradeoffs do exist in the theoretical model but don't seem to in practice then: 

• Practice is not actually based on the explicit theory but is instead based on something else, or
• the tradeoffs do in fact exist in practice but are not noticed or acknowledged. 

Both of these would be foundational problems for a movement organized around rationality and  evidence based practice.