I completely agree with your second paragraph.
To your first paragraph: yes, it isn’t obvious that intelligent life should be easily visible. I certainly don’t think we’d be able or likely to detect a human-level civilization even a few star systems away with any reliability. I don’t care about that. If intelligent life is common and there is no future great filter, there should be many civilizations billions of years old even within the Milky Way. Am I really supposed to believe that none of them have ever done any serious stellar engineering? That none of them colonized the galaxy with generation ships and their equivalent of terraforming?
I don’t care what your biology is, not eventually building billions of Dyson swarms in your home galaxy is at least an interesting choice (either there are totally new principles of physics that make it unnecessary or else the one species that comes along and does it will outcompete you). And that would be visible. There would be vast numbers of apparently dark or infrared or weirdly inconsistently bright stellar-mass entities, and there aren’t. It still reduces to the same set of options: no aliens (early great filter), aliens extinct (future great filter), aliens deliberately hiding, or amazing new physics causes 100% of aliens to never undertake large scale visible stellar engineering projects even in civilizations billions of years old.
Intelligent life on Mars would basically eliminate the first option, which necessarily makes the second more likely, and that is the one outcome I really, really don’t want to find.
