Considering how usually the first game in a series that first released on the NES ages quite poorly and doesn't really need to be revisited as the sequels vastly improve on the ideas introduced in the first game (this is very evident in the Mario, Metroid and Zelda series) I think Mega Man 1 is still a fun and short time and for the most part it doesn't really feel outdated, although that's likely because 8/11 of the Mega Man games all share the exact same NES graphics. There are definitely frustrating parts which can make this game feel outdated, but in my experience of playing Mega Man 2 and 3 they are equally as frustrating at times, so I don't think that's a problem with the game and more a product of the time where games were generally harder.

Overall, whilst I can see why people may not like this game too much as it's a bit of a clunky NES game at times, I think it's a good introduction to the Mega Man series as it presents the general idea of what you'll see in a Mega Man game, as well as providing very catchy music and it's short enough to where it makes me want to continue over to Mega Man 2 where they start to improve on the ideas in this game.