2 articles are attached. Whilst in young people there can be significant over-eruption ("dropping down"), this lessens for older people. These are 2 (admittedly small) studies with ages 33-58yo and 25-69yo which conclude that there is clinically insignificant movement in adults. Although the absolute age of each person is not cross-referenced to the amount of overeruption, I think it could be safely assumed that the older the person, then the smaller and less significant the movement.
In other words, you probably don't need an implant. If your dentist suggests otherwise, then you have these articles that suggest otherwise, unless he can find other literature.