My son, nearly 3 years, has sucked his thumb chronically since 4 months old. Hasn't hindered his speech at all and he is quite chatty, pmays happily without sucking thumb, but going to sleep and any quiet time, there's the thumb.
At his most recent dental nurse visit, she said it was causing quite bad protruding teeth. She said us some children there will be permanent damage (ie to shape of jaw development etc) but in some children as they grow up their teeth will be fine. So there's no rule that says all thumb suckers will be in orthodontic treatment in the future.
But she did recommend we try to deal with it. At under 3, she thought too young for rewards etc as the urge to suck is too strong to outweigh rewards. Thumbguard product was recommended - it looks horrendous and trying to strap a child into it can't be easy, and it us expensive (not as expensive as braces!).
So, while I was pleased when he was younger that DS had a means of self soothing , I am not so thrilled now.
Hopefully yours grows out of it sooner than mine has! And giving up a dummy is evidently no picnic. No advice really, maybe as yours gets to 12 months try more distraction or get advice then about discouraging the habit?