Legitimately every villain in this show proclaims that, "I'm gonna take over Equestria, evil laugh". Yet, they don't destroy the mane 6 right away, they start rambling on and on and literally spill out their plan to everyone with revealing dialogue, pretty "clever" right?. None of the villains think about being reclusive or sneaky. (Then again, not every villains goal is to ambush the mane 6 and kill them, it's specifically to overthrone
The Dazzlings: "It's Equestrian Magic!" *mind controls everyone under their spell", when they should've realized The Rainbooms would defeat them anyway, (despite being trapped underground; someone eventually could've opened the door for them, but Adagio totally ignored that possibility), because they're the only ones not under their spell.
Nightmare Moon is basically the corrupted form of Luna, but again, she laughs maniacally in the first episode, taunting the ponies, and scaring them without actually harming them, and she vanishes, only to see she makes a move on the mane 6 in s1 e2. Maybe she wanted to give them a warning and "play with their emotions first" cliché.
Discord, a former villain, also turns the mane 6 gray and apart from each other, clashing amongst their friendship, if he wanted to succeed wouldn't he destroy them all, but then what would be the fun if there were no good guys to turmoil with, maybe? Plus they wouldn't kill off the mane 6 and other characters right away, that'd just be another cliché.
Tirek wanted to steal all the magic in Equestria and rule as its lord. He doesn't kill his enemies, rather preferring to leave them alive. But doesn't he know if he doesn't assault the mane 6/everypony else to their death, he'll fail eventually?
There are many more villains, but I'm too lazy to put the rest in. The point is the villains are so arrogant and stupid that they want to conquer everypony to watch them suffer, but haven't they realized that to defeat the ponies, they could get rid of them while they're asleep? The villain's tactics and poisons are super weak, since the outcome is that they always lose. Yet they keep coming back using the same abortive attempt to overthrow Equestria.
And of course it is a kids show, why would they kill off anypony? Especially the main characters, that to. In essence to that, they do attempt to "kill" the mane 6, but they never die since their magic is so strong, and there are more protagonists than antagonists.
Moral: Clichés will always be there in different forms, but some are vastly greater than others, and villains aren't very sharp and quick on their actions, because they're so absorbed in their own thoughts and fantasies of how great they are.