My D&D character Walker made 20th level last game. It is the first time I have played a character straight-through, legitimately, levels 1-20 in the thirty years that I have been playing D&D. Through version 3.5 level 20 was the final "regular" level of D&D. Guidelines for levels beyond that were generally considered optional or expansions.
To people who don't play the game, well I am sure they could care less (lol), but if they did care, some of them would probably be surprised. I think they would be surprised because they are used to games that have a single goal or objective, that you "win".
Most role-players will probably not be surprised. D&D is not such a game. You could have myriad goals for your character, acquiring certain items or getting certain abilities that have nothing to do with leveling. They might be role-playing goals or they might be mechanical. You might start your character after first level. You might end the campaign your character is participating in before 20th, the edition of the game might change or you might get interested in a different role-playing game. Your character could die. Your gaming group could split up.
Staying with the same campaign and characters for so many levels takes a long time and a lot of commitment. We have played consistently once a week for the last 5 years. We took some breaks along the way, maybe as long as 6 months, but we kept going. Notice it's all we. This is a commitment that has to be shared by about 5 or 6 people, come hell or highwater. No matter how much people want to play it can be hard to make the time to play.
I didn't really think about it much up until the last few months when it became clear it might happen. Then it started to seem like a great achievement.
I am happy and proud that we made it. Seeing these characters live has been a great thing.