So continued existence can be either involuntary or irrational, and suicide rates can be low even when life generally feels more bad than good. If all sentient entities could become rational decision-makers whose conscious existence is universally voluntary, that would probably be the most significant improvement of life on earth since it evolved.

I agree. See also this comment and subsequent discussion. I consider low suicide rates to be weak evidence that people find their lives worth living, not definitive evidence. There's other evidence, in particular if you ask random people if their lives are worth living they'll say yes much more often than not. Yes they may be signaling and/or deluded, but it seems hubristic to have high confidence in one's own assessment of their quality of lives over their stated assessment without strong evidence.