Some counter theories: People have fewer kids not because they drink less, but because children are extremely expensive, wage growth has been stagnant, women make up a larger share of the workplace (and don't want to lose their spot), and birth control is more readily available.
People are lonely because: women are capable of larger roles in the workforce and it's upended social expectations, therapy has caused people to avoid others that they deep destructive/negative in their lives, technology has caused us all to be alienated, or society has shifted but culture hasn't caught up to these changes (assuming they're lonely at all and not just experiencing the same existence previous generations did but now with constant news cycles and the ability to tap into 24/7 feeds of people having fun).