'what if' my eyes did remain stable through my teens, twenties and so on? Does anyone have any experience in this?
The reason why I mentioned the deteriorating trend was in fact from my personal experience.
Thankfully I am still able to hold the Class 1, however had I started with close-to-the-limit eyesight, I would not have. Throughout my teens I maintained the eyesight where I did not require glasses for anything. However at around 20 years of age, my eyesight dropped by around 1.5 dioptres. Then it stablised for a while. Then it dropped further between the age of 25 and 30 by another 1-1.5 dioptres. No cause is known in my case - it just deteriorated. Fortunately, it has now been stable for a while. However, while it was deteriorating, I would have been quite worried had my eyesight been close to the renewal limit.
Of course this is all extremely individual and you cannot say that because other people suffered from this and that, you too would.
Asking your optometrist on statistical data on eyesight deterioration may give you some insight into the issue.