There are tangible benefits of having numerous children, but they tend to take a while to materialize (ex: a lifetime). More kids = more siblings. This can be a ginormous benefit to children as it provides them with a built in social support network they can grow with over the lifespan (provided they get along decently well that is). I learned so much from my older siblings as they went through high school, college, and early adult life before me. When you get old and deteriorate, they will have others to lean on for emotional support as they try to navigate such a difficult transition. As you yourself age, you will have more children around to visit with and engage with – children and grandchildren are a critical social support network for older people, especially as their friends and family begin to die away. More kids generally means more opportunities for grandchildren, family holidays and vacations, people to come with you to scary doctor’s appointments, etc. Don’t discount these benefits as they can mean MASSIVE quality of life improvements later in life. Of course, none of this is guaranteed but then again none of the “Benefits of kids” or “benefits of being childfree” are really all that guaranteed either. Generally, if your kids are healthy and neurotypical without any serious issues (personality disorders, addictions, etc), and you do a good job raising them to be loving/compassionate/smart people, they will end up with a healthy family dynamic later in life.
Every day I see the huge dividends that having multiple children have bought my parents, inlaws, and grandparents. Our parents are now in their 60s and their social calendars are packed with family activities and playing with grandchildren. Even though many of their friends have moved away or are busy with their own families, they continue to have high levels of social engagement and inclusion through their own large families. On numerous occasions my mom has insisted that having her grandchildren and knowing they rely on her to be present in their lives is what keeps her young, and also what helps motivate her to stay fit and healthy.
Obviously more kids is generally more expensive and time consuming. But I do think that our current culture often obsesses about the shorter term costs of children rather than seeing children as a resource across the lifespan. Sure, they have some crazy high upfront costs, and sure you could definitely end up in the extremely difficult situation of having a child with serious mental or health needs, but healthy, well adjusted and loving children who are engaged with their families often pay for themselves ten times over decades down the line.