There are so many variables here it’s impossible to tell you what the right thing to do is but there are a few factors to consider
implants and bone grafts have a much higher failure rate in those who smoke, vape , have poorly controlled diabetes , are on certain medications eg bisphosphonates (for osteoporosis ) . If any of these apply to you then you may want to consider sorting these factors out , or where medications are taken that can’t be changed this might affect your decision .
Bone grafting at the front of the mouth tends to be more problematic than grafts at the back because of the type of bone defect. As your dentist has said there is no guarantee of success of this or indeed any dental treatment , so you need to be sure that you are happy to undergo treatment knowing this. After 20 years I would imagine there is quite a bone deficit where the teeth are missing . If you do go down the graft route I would consider asking for sedation which will add a bit to the cost but will make the proceedure much more pleasant for you.
Have you any other teeth missing at the top? Many people have plastic dentures as a temporary solution to missing teeth and have huge problems getting on with them. Metal framework part dentures, chrome cobalt , are much better fitting , thinner, don’t normally have a palate in and are much better tolerated. They might well be worth considering even as a medium term solution , and if you have other teeth missing will solve that problem as well. They can last many years before needing replacing .
When you drill teeth down eg for a bridge or crown , there is a 20% chance that the nerve will die off , which means the tooth would need root treatment . A tooth that is root filled is a much more brittle tooth and potentially has a much shorter life span . On average a bridge has a 10 year lifespan although they can last decades longer . A Maryland lasting 20 years is very,very good so it sounds as if your bite and cleaning will not be a problem. The other factor is how good the roots are on the supporting teeth and how good your gum health is.
In the end it’s down to what your attitude to risk is , as , apart from the chrome cobalt denture option, both graft and bridge carry an element of risk of failure/complications .