Awwww...here's the rub...
Ideally, people want the game to emulate the kind of stories wherein the heroes face seemingly insurmountable odds, yet somehow manage to eke out a victory despite all probability.
The problem is that D&D is a game, and not a story and this concept is untenable because...welll...that's how probability works. If the system consistently stacks the odds in favor of the opposition, then you can be reasonably sure that the PC's will nearly always be on the losing side.
As for the specific problem of level drain and ability score decreasing or increasing effects, I agree that these things have a place in the game, but that the classical implementation of them needs to be streamlined.
This D&DN experiment haslofty goals...some would say unachievable..but it will never, ever actually work if people aren't willing to open their minds up to new ways of doing old things.
I'm usually playing for a balance of game and story, so the rules are placed well behind player choice. I see the problem when the rules come first and the rules model or favour certain types of outcomes.
Rules rule doesn't really crop-up round the table, as there's an understanding that if players play well, have a go, get into the game . . . everything will balance out, e.g. a PC might have lost a level, but the same monster's hoard contains a prized item the player has always wanted for that PC.
I have ability loses for poison and drains as a good option in AD&D, but wouldn't have the slightest concern about another approach that still scared PCs and was consistent with the notion of undead, e.g. the difficult to heal damage of a mummy could be varied to give a chilled to the bones damage effect like Frodo getting stabbed by the wraith.
All for new ideas - and my only 5e disappointment so far is the statement about not including new trends in gaming. It appears the design is aimed at more of a re-visiting than a re-invention. That said, if the core is modular enough they'll be able to stick anything on it, maybe allowing novelty to win out in the long run.