I guarantee you that if you played Rayman 1 instead of Legends, you would probably say the same thing as I have.
I did play Rayman 1, and I never really liked it - much like older Mario games, I don't have sufficient interest to invest all that time in a challenging game. In fact, I have pretty muhc exactly zero interest in bashing my head against difficult levels in *any genre*, or wasting time farming lives in easy levels so that I can tackle the challenging ones. It's just not a part of gaming that is worthwhile to me.
Not all gamers are the same - if they added lives to Rayman Legends, you might be happier. I, on the other hand, would be less happy. Which is why I have to reject your blanket assertion the 'all platformers are improved by the addition of a limited lives mechanic'.
While I do agree that Rayman is hard, the Mario games can easily be beaten with just using a power up that can be achieved. Infact the games are well balanced that in world, even if you only want to get a specific power ups, you can manage to stock up extra lives in the end.
If you love Legends, then you should have noticed its game design flaws by now. The problem with legends isn't just not having lives in them. The problem appears on the other design aspects of it. Making the characters incredibly gullible for no reason when the previous games added humor at reasonable points of time. But this one tries to make humor everywhere and its executed poorly. Then comes the problem with the level design where everything is just given to you.
Hidden doors are always noticeable when the point of them is to be well hidden and as a result, fails to motivate the player to explore much. You might think that the player might be fine with this but the problem is that you need to get certain collectibles to unlock more levels and as a result, it frustrates the players more forcing them to complete everything as much as possible instead of them playing the game straightforward. So the game forces you to get the collectibles which should instead be optional. So what does this have to do with the doors not well hidden? Its something that the player cannot ignore if he wants to make more progress so obviously the player is forced to go to every hidden door to get items which again are needed inorder to make progress.
I love the way how the original Rayman allows you to go to newer levels and then requests you to 100% the game to give you the best ending since adding more content is time consuming, a good ending would suffice and that is good game design. Giving players satisfaction regardless of the reward is important. What does Legends give you in the end? Pointless items that are better off as achievements than the actual game items. Sure, you get more levels if you give it to them, but a good design would be to make it optional and head to the next level.
You can argue that some games like Super Mario 64 or Rayman 2 did what Legends did but here's the problem. They are 3D games. The worlds are bigger and the levels are in the form of short tasks that easily motivate the player to explore more of the levels than actually being forced to get requirements to go to the next because you haven't experienced the whole level yet so it is a good game design to divide the level into small sections.
While its not the same for Rayman 2, the collectibles are always in front of you for the most part so getting them is a breeze. Legends does the exact opposite where instead of just breezing through the level, it forces the player to explore more to get rewards which are barely satisfying.
I agree that not every platform game is hard and not every one loves hard games but a level of discipline is needed inorder to enjoy a game. Again, what is the point of not having lives when they are easy to get in the first place.
I played Ducktales on medium and I had a fun time with it. If I played it on Easy, its bound to be less fun with no lives.
I gotta say, Kid Icarus Uprising did something very unique where the player can adjust the intensity of a level to get either less or more rewards in the end. Its just a shame that its not a platformer and even then, the sections are not well designed. If platform games were to have this mechanic, then I would agree that having lives would be pointless.