Many parents sleep training their first child will hesitate when I explain to them that 6-6 or 7-7 is normal for young children and babies. They do not want to get up at that hour, they are used to "sleeping in" with their baby or toddler next to them (albeit sleepness nights), and they simply have the luxury of staying home nad doing whatever they want in the morning.
But that isn't the reality for working parents, families with other older children, or for babies/toddlers who are naturally early risers.
Not to say that having a "lazy start" in the morning isn't the dream (trust me... I would love to sleep until 8:00 AM every morning!!!) but babies and toddlers do become hard-wired into routines- for better or worse.
When you begin sleep training, children are often overtired and need an extra boost in sleep to recover their sleep debt. This is referring to the lost sleep they've accumulated from frequent night wakings, napless days, and so on. An earlier bedtime promotes stronger rising melatonin naturally built into their bodies.
What about families who swear by a later bedtime, because their kid thrives from 9-9 or 10-10? Sometimes I even see families suggest they do 11-11 beautifully with their family.
But my main concern with this model is that it isn't realistic for the long term. Eventually, toddlers become children who go to daycare, Pre-K, elementary school at 5 years old at the latest. The years leading up to that are an important time to create long lasting habits. It takes a long time to re-wire your child's brain to want to go to bed earlier. If you don't start long before the early morning routine is necessary, you will certainly struggle with pulling them out of bed in the morning. Early starts as young babies and toddlers can have impactful and long-term benefits as your children continue to grow up.