I mean yeah, it's irresponsible to let people go outside, drive cars, play sports, consume media by personal choice, engage in romantic relationships, climb stairs and ladders, drink alcohol etc. Yet we still generally feel the quality of life improvements outweigh the quantity of life decreases.
Consider also that humans confined to an indoor environment have much more environmental enrichment available in books, television, internet, games etc. The only environmental enrichment available to cats indoors are really terrible simulacra of nature (strings, balls, furniture) and many of these are only available when people are available to play.
We're talking about a species that spends 2/3rds of its time napping. I've watched outdoor cats and their behavior (when not hunting, which not all cats do) is very similar to indoor cats. They nap, they wander around, they find another napping spot, and so on. They probably spend less time interacting with humans or other animals than indoor cats do.
Consider also that humans confined to an indoor environment have much more environmental enrichment available in books, television, internet, games etc. The only environmental enrichment available to cats indoors are really terrible simulacra of nature (strings, balls, furniture) and many of these are only available when people are available to play.