I just don't think it's a fair criticism to say that D&D 5e's mechanics make it less deadly than other systems if one is willing to take a perfectly viable means of threatening the PCs off the table.
It's not like you couldn't attack downed PCs in prior eds, either. In those editions, there was a -hp death threashold, and those negative hps added up, in 5e you get a failed death save, instead. So, at least two hits while you're down to finish you, if you haven't already failed a death save, 3 if you're constantly making your saves. In 1e, if the DM didn't have you die outright at 0, you died at -10, and it's not like it's hard to do 10 hps in one hit. 2e, IIRC, and 3e automatically used the -10 hp threshold, too. So they're all deadlier than 5e when you're down and the enemy is beating on you...
OTOH, 5e's instant death rule is negative your hps, which, if you're 1st, just might be less than 10. So you can be insta-ganked more easily in 5e, at 1st level (maybe even 2nd if your CON sucks). FREX, a 1st level magic-user would have had 4 hps in 3e-and-earlier, and could go to -10 before dying (for a total of 14 hps between him and instant death), while in 5e a 6hp 1st-level wizard is only 12 hps away from instant death even when fully-healed. OK, 2hps isn't a /big/ difference, but still, you can so totally die in one hit. The same applies as long as you have fewer than 10 hps, so apart from the tougher melee types, everyone dies easier in 5e than any prior ed,
at first level.
That's gotta count for something in terms of being 'deadly.'