In my experience, babies will get enough sleep one way or another, with or without "sleep training", though it may not be on a consistent or convenient schedule.

Parents can easily get enough sleep as well. It's called "sleeping when the baby sleeps". Infants sleep like 15 hours per day, so it isn't hard. You just can't conform to any kind of a schedule--that's the tradeoff, not getting enough sleep.

I think you're wrong about the first part. It's probably right for newborns but not even older infants and it is definitely possible (even extremely common) for toddlers to get too little sleep if left to their own devices.

"Sleep when the baby sleeps" sounds good but is not really how sleep works; sleep quality is very sensitive to cycles and time of day (it's also a very personal thing, with big differences between individuals). It also doesn't work at all for working parents. And even in households where one parent doesn't work, the other one does, and it is important for both parents to be rested.

Of course you can pull this off, I'm not saying anybody is going to die or anything, I'm saying there are very real advantages to more predictable and consistent sleep. For us, we were very hesitant (or maybe just lazy) about sleep training, but our ~15 month old was noticeably happier after being able to sleep through the night.

> Parents can easily get enough sleep as well. It's called "sleeping when the baby sleeps".

I did this with a puppy (when they're little they'll wake you every 90m at night) instead of putting her into a cage and once you figure it out it's really not that much worse than sleeping through the night. Perhaps doing that for a few months is measurably different from doing it for a year or so, but I seriously doubt it as chronic sleep deprivation (which I would have had if I had not adapted to the schedule) starts being very harmful very obviously quite a bit before "a few months".