Thats sounds more like armchair design rather than actual in game observations.
If you mean it sounds more like a simple mathematical analysis given the rules as they actually exist than an unsupported, unverifiable anecdote, you're right, that's exactly what it is.
But, hey, if you like anecdotes, I did run the playtest through three encounters seasons, and had a number of character deaths at low levels.
The first was on the second initiative count of the first round of the first combat, when a character (under the then-current packet) failed a save to avoid taking 1 hp of damage and was reduced to negative max hps and died, before anyone could stabilize her. The scenario was simple, a combat started, we rolled initiative, the mind-controlled NPC went first, the rogue - whose player has spent hours putting together a character with the noble background and coming up with backstory for her - second. The NPC critically hit the rogue, knocked her to within one of death, and the rogue died on her turn. Two rolls went against the player (statistically, a ~1:40 event) and the character died. More, though less sudden, deaths followed.
MiBG really was murder. A cleric and rogue both succumbed to the phenomenon of a moderate damage roll knocking to very low hps, followed by a high damage roll killing. A second cleric was 'assassinated' - the enemy was specifically out to kill her because she'd openly betrayed one of the major players in the de-facto thieves' guild, and hit her while she was down, killing her by inflicting failed death saves. The replacement for the first cleric who died also died, again, in much the same way, and player opted to finish out the season at a 4e table. Oh, and there was a TPK.
OTOH, the season I ran that started at 6th level had no characters killed, nor even dropped.
I'll be running 5e next season, so I'll have more anecdotes for you late next month.
Essentialy you are taking an extreme situation and treat it as the average.
My intent was to present it as a 'worst case' scenario on the PC side (1d6+1 is hardly over the top for 1st-level monster damage). I'm sorry if I didn't make ' hp character' clear enough. That's a 1st level, 10 CON wizard. Most characters are going to have more hps than that - but a character /can/ have that few hps, could even have 5. The 'instant death' rule quickly goes from unlikely to virtually impossible as you add levels. By 3rd (when most 'heroic' games should start), it shouldn't happen short of some kind of fluke like a /very/-high-damage attack on very badly wounded character.