True that having a harder mode makes it challenging but here's the problem.....it is only suitable for getting achievements rather than playing the game again.
I'm honestly not sure of where playing the game again comes in: I'm talking about one's first play-through. If you enjoy highly-challenging gameplay, then at the start of the game you select the "hard" difficulty setting and enable "limited lives". If you prefer unlimited lives, or just want to play something light, you select the "easy" or "normal" difficulty setting, and disable "limited lives".
Encouraging replayability is another question, and can be addressed in various ways, including challenge modes, unlockable items, and the like.
Well if the kirby games were really that easy, then they have to like put some level of discipline to make it worth a challenge.
Are the players of Kirby necessarily looking for a challenge?
I don't see any harm in them nor do I see any issues in them since it is some discipline that the player needs to have when playing these games.
Why do they need to have that discipline? Why can they not just have fun? (I'm not saying that it's wrong to want or have that discipline, just that some people might be looking for a different experience.)
For example, what if a player enjoys the activities inherent in platforming--running and jumping, collecting items, perhaps killing enemies--but doesn't value challenge as you do: they just want a fun experience. This player wants to see the next bit of level, the next hurdle, enemy or set-piece, but isn't interested in having to watch their life-counter. To them, enforcing a "limited lives" mechanic might harm their enjoyment of the game. Is their preference invalid? Are they not allowed to have games that they like?
What about someone who wants some challenge, but doesn't have the time to master the mechanics? Being shunted out to the main menu might be frustrating for them.
Something that I've been trying to suggest is that what you value in a game--challenge and mastery--isn't necessarily universal. Come to that, your level of mastery, and your degree of interest in attaining that mastery, isn't necessarily universal.