If the DM presents a scenario where the players have a chance of stealthily taking out Sentries, then the Combat rules may not be the best way to resolve this particular challenge. Depending on the approach taken by the player(s) it could be resolved with a single Stealth(dexterity) check or maybe even no check at all if the DM determines their approach is so good it's an automatic success.
While it may not be common 5e does support this style of play too.
I think the reason it is not common is because anyone who knows the rules knows it is forced. Anyone who wants that risk knows that all the risk is against the player if they take the action. Therefore, there is never a reason to attempt the dramatic action. Some adaptation of a Plot Point system could be added, but that still eliminates the die roll. In much the same way that all video games are really just elaborately skinned versions of Pong, all RPGs are an elaborately dressed game of "shooting dice" with improv interpretation of the roll as the reward. The random element is I think essential to the game, with a referee to maintain the suspension of disbelief by editing out the absurd. (A fire elemental catching a water elemental on fire being an example.)
It occurs to me that a house rule could be effected as follows:
Player of Ranged Character: I use stealth to get within range to attack the tower guard. (PC rolls 23)
DM (Rolls Guard's Perception): You move within 40' of the 20' tall platform and now have a clear shot within range of your short bow. (Guard rolls 8) You have surprise.
Player: I roll 21 to hit.
DM: That hits, roll a d6 for damage and add your dex mod.
Player: I have 6. Exploding. I have 4. Explode again. I have 5. And again. I get another 5. And a 3. I have +2 Dex; so 6+4+5+5+3+2=25.
This could be used equally with melee character and applies for anyone on surprise round. This would dramatically increase the spike damage of a Rogue, especially Assassin, in surprise rounds. However, there has already been a suggestion that their spike damage as is may be more at issue. Since the math used is simple and from already existing rolls, the only time delay is rolling the additional damage the stealthed character makes.
This also makes stealthier weapons more effective for stealth attacks. Exempli gratia, a dagger is stealthier than a great sword. The ease of making such an attack with a dagger, using our numbers above means the dagger is more likely to explode for a lot of damage as it quietly slits a throat or jiggles in a spleen. A great sword is harder to wield so precisely, and doesn't explode as well. (Then there is history, such as the real great swords - flamberge, never even being sharpened but used as clubs to knock people off their horses and then club them to death by crushing them with their own bashed in armor.)
I may house rule that in as it adds a little stealth to everyone and meets the story dynamic for which I am looking. It dramatically increases both Sneak Attack and Assassinate on the first attack when the target is surprised (does not know the Opponent is present and is not prepared for an attack). I suspect only test play can determine if the trade off is worth it.
Someone earlier suggested an exploding die option. This is my summation of positive possibilities for a mechanism so far.