The article focuses on this issue when thumb-sucking begins to affect secondary teeth, as hammock and the article point out it becomes an issue when the behavior is retained with secondary teeth. 3.5 years seems like he's reaching the threshold for when to ween.

"There can be downsides to thumb sucking, as it can lead to problems from pushing secondary teeth as they come in. But those don’t come in until later, when children are in elementary school. Most children stop sucking their thumb by themselves before they get to elementary school. The American Dental Association recommends that parents start encouraging their children to stop sucking their thumb after age 4 — but even then, they advocate a gentle, positive approach. Many children suck to relieve anxiety, and making them anxious about sucking can backfire! Instead, praise children for not doing it — and help them find other ways of soothing themselves. Putting a Band-Aid on the thumb can help them remember; at night, using a sock or something similar can help with the child who sucks their thumb without realizing it. But again, this is only an issue for older children."

> The article focuses on this issue when thumb-sucking begins to affect secondary teeth

Which ignores that fact that crooked teeth in young children can impact speech development.

It can also create breathing problems like snoring in later life

Luckily all his teeth will eventually fall out

Interestingly, the position of the baby teeth greatly affect the adult teeth. As it was described to me, the baby teeth are essentially placeholders or spacers that reserve room for the adult teeth. If they are out of alignment or move to funny positions, your adult teeth end up in odd places.

My little brother was starting to have the same problem as his secondary teeth were coming in when he was five years old. The dentist sat down and simply told him: "if you keep sucking your thumb, your new teeth are going to come in crooked and it's going to look really bad, and we are going to have to do more adjustments and visits here". My brother stopped sucking his thumb cold-turkey.

Granted, he's always been a determined kid :)

FWIW, we just weaned (as in, this week) my 3.5yo daughter. That nasty nail polish stuff worked perfectly. Took her a little longer to fall asleep the first two nights, but now it's no big deal.