I dislike the idea that "no one should be able to defeat a prepared wizard". Why not "no one should be able to defeat a prepared fighter"? Why is being awesome through planning and preparation the sole domain of magical classes? I could never understand that.
I think "no one" or "never" are probably not quite the words I should have picked. All characters and classes should see their odds improve with preparation. I just like the idea that wizards are even more so. That also fits the 1e flavor of fighters vs wizards, where it took more thought and experience to play the wizard "successfully".
You can argue that it isn't fair that you can't play a magic-user as a newbie or that the fighter doesn't give advanced players enough options, and that's a reasonable argument. It doesn't change the fact that some players like to have more complexity to their characters and/or take a strategy-over-tactics view of the game. Making the game conform to the tactical, combat-as-sport mindset is no more fair to those players than forcing tactically minded people to play a strategic character.
I really think the answer is to recognize that there are tactical ways to play spell-slingers and strategic ways to play sword-slingers. You can have fighters and wizards both be good at both, but it's unfair to have fighters only good at tactical situations and wizards good at both. What makes sense to me is to have different classes emphasize different aspects of the tactical/strategic axis. This also seems to fit the modular and "different rules, same table" ideas that seem to be core to 5e.
That doesn't mean that you can't play a tactical caster or a strategic warrior. It means that you include this in class balance and the player picks a class that best suits his style. So, the person who wants a tactical caster chooses a warlock, warmage, beguiler, or whatever it is. The person who wants to play the thoughtful magus who can slay the dragon in the first round, but only if properly prepared, plays the wizard. There's also room for a middle ground, but I'm not sure any legacy arcanist fits that bill. Maybe bard or sorcerer, but that could be a flame war in itself.
Do the same for the martial characters. Personally, I love the idea of the ranger as a martial character who can go all Chuck Norris on your butt, if he gets to control the situation. Give them options for prepping their environment, studying foes (more assassin death strike than favored enemy), and stealth. Then don't worry about the fact that they wear lighter armor and don't have weapon focus, TWF, or a stupid-high bonus with a bow. They're aimed at more strategic players and using them tactics-first should end poorly. The fighter could exist at the other end, but I actually see something like a swashbuckler/duelist or barbarian (neither of whom are noted for deep planning, as archetypes) be the tactical focus. Let the fighter have the golf-bag-o-weapons, some stances and/or maneuvers that take a few minutes of meditation to swap out, and some extra healing surges or action points to spread over the day. That should put them in the middle, maybe slightly on the tactical side. The warlord might be set up to fill the middle-to-strategic position. The rogue should probably be extremely strategic in nature -- maybe it would be a good idea to drop the "rogue" name and stop trying to have one class fill both the thief and duelist roles.
Again, I'm definitely not saying that you can't play a mage unless you want to play a "prepare or die" character. I just want to recognize that there is are different play styles and the wizard has evolved from the 1e magic-user which was, very decidedly, in one camp. Leave it there, but give us something to fill the void. Nothing more. Nothing less.