That's a lot of assumptions there, all of which are going the pcs' way. You're assuming someone in the group has healing word and spell slots, that they aren't going to use their bonus action for something else, that there's only one attack available to spend on the fallen pc, that- once healed of around 5-10 hp- the pc won't just drop again in one hit, etc. These things aren't always true, and they are rarely all true. Or if they are, your encounters aren't using enough monsters to be as challenging as you want.
Seriously- if you want a more lethal game, even without changing the makeup of your encounters, it's easy to achieve. Even if the pc gets saved, you have taxed the party's resources more, used up some of their actions, etc., making the fight harder. Tactics are a far more effective way of buffing an encounter than some folks credit; they can change something from a difficulty of "easy" to a difficulty of "holy crap, run!!"