Also, having replayed the first Castlevania this week, it is awfully short, and that's probably because of the difficulty — you do get infinite continues though, and must restart from stage 1 only if you reset the game — because if you know your way around, you can deal with it rather fast. The other episodes are longer but have a password/save system, as DR said, probably to compensate for the increased lenght. That's pretty much a good trade-off to conserve the challenge — when it comes to Castlevania 3, 4 and RoB anyway, because 2 is damn easy. Same thing with Super Meat Boy: you may have infinite lives but there are many levels and secret zones, many that will take a long time to complete. Clearing 100% of the game isn't a week-end relaxing affair, and you do not have to if that's not your thing, but for those who like the challenge, it's there. And for the masochist, there are the iron man (boy?) achievements, which are no small feats. The challenge may be of a different kind from older games, but it's still a challenge nonetheless, and parts of it are optional. Just like for older games, you do it because you want to, and get a higher score, an achievement, extra ending, or just nothing. On that note, just because most achievements in games are trite doesn't mean it's a bad idea.
In fact I'd say that games like SMB tap into old challenge design but adapt it to modern sensibilities. It is designed to offer a lot of small chunks that you can tackle in the order you want, and even then just completing one of these chunks doesn't mean you can't come back later to improve your performance. The bad thing is to take an old design as is it and give it modern features, like quick-saves. You can already see the result by playing Castlevania on an emulator and using save states. Doing away with old conventions isn't a bad thing as long as the gap they leave is filled in some way.