I personally enjoy the large variety with gameplay. It's much better than, for example, having 30 assassin type characters who all have different combinations of the same 10 skills. When something new is introduced, people are forced to learn and adapt the way they played previously. This goes for items, new champions, reworked champions, different modes, and whatever other changes you can think of. Every champion has their own gimmick, and through not every champion is able to walk through or jump over walls, they each have things that other champions may not have. There are ways to avoid most things that would one shot you, and if you feel you can't avoid it, you can build to counter it. You have to adjust your playstyle and build according to who you're playing with and against.
Not all champs can teleport. Not all champs can jump or walk through walls. Not all champs can create their own obstacles to block others. Not all champs can heal others. Not all champs have good ranged poke. Not all champs can dive their opponent and make a good thing of it. Not all champs can charge or leap on their enemy. Not all champs can stun, snare or fear their opponent. Not all champs have a massive radius/range on their ult. If you want to argue against what some champs can do, prepare for a massive debate of what that champ cannot do compared to what other champs can do.
If getting one shot is upsetting to you, play tankier. If you're being one shot, it's probably because you and your opponent are both fully investing into damage, and you ended up getting caught or being splat first.
Most new champions on release do seem a bit overwhelming, but it's often largely due to people not knowing how that champion works. They may not know what skills they have, and therefore don't know what to expect until they see it first hand. In my personal opinion, a lot of the new champs don't seem nearly as op as you're trying to make it sound. In most cases, I've seen them either succeed, or fail miserably.