What you choose to see as blindsided and crippled others may see as an opportunity for adventure!

So now you have the three amigos down on hp maximum and desperate for a way to get their mojo back. There is a legend about a magical fountain to found in the nearby mountains. They say that those worthy enough to reach the fountain may benefit much by drinking from it.
Take lemons- make lemonade.
This would be peachy-keen except that I'm sure you can imagine hundreds of possible scenarios where such an adventure would not be a welcome intrusion into the game, and the key point here is that if 5e is like this, it gives you
no way to know this without some pretty in-depth system mastery, so you can stumble into this without wanting to or intending to. If I've got the space for it or I'm aware of what I'm getting into -- if 5e telegraphed that parties without healers might face this -- then I could still have the opportunity for adventure, but I wouldn't be caught with my pants down when it came up. I could consciously choose it. That would be what explicit roles do.
Plus, I ordered the lemonade, I'm paying for lemonade, I want the lemonade, you can't give me lemons and pretend like it's good enough because I can squeeze them and mix some sugar-water in myself if I want lemonade so bad.
All that was done was cosmetic name changing. Just replace whole groups of specific classes with a role and you have the same stuff. Rather than waste valuable pages basically telling you that different classes have (GASP!) different strengths & weaknesses, the 5E team chose not to assume that the average interested D&D'er had been lobotomized and that a read through of the classes would paint a decent picture of what each class was and wasn't capable of.
There's a LOT of space between "strengths and weaknesses" and "you are screwed without this specific spell." A read through the PHB will show you that different classes are different, but will do nothing to show you that you MUST have the
greater restoration spell in the party or you're going to be hosed with certain encounters. Is there some encounter out there that will hose me if I don't have
fireball? What about
Bigby's Hand? Why would I expect
greater restoration to be any different from any other spell in that it is a useful tool but not a necessary bar for being able to fight the thing?
It would be like if I made a monster (call it the Puccini) immune to everything except +2 bohemian ear-spoons and then told the players that if they didn't anticipate having to use a +2 bohemian ear-spoon that this is THEIR problem for being "lobotomized" enough to somehow not divine how important a +2 bohemian ear-spoon is in D&D. Clearly they deserve to get crippled because they forgot to bring one into the dungeon that had the Puccini encounter on the random encounter tables and then decided to fight it WITHOUT a +2 bohemian ear-spoon. Not my problem, right? They'll learn next time!