Late to the discussion and there's so much to cover...
Spells don't scale to address HP bloat at later levels.
That's when you start using your caster's Maximize and Empower feats. I don't remember all of the spells in D&D, but I do remember that there are several spell levels that don't have any damage-dealing spells. So you may as well fill those slots with lower-level maximized and empowered damage-dealing spells.
Also, like someone else pointed out, straight-up HP attrition is a tactic for chumps. There are so many more effective and fun builds and tactics out there. One of my personal favorites was the
Spike Chain Tripper, A.K.A.
Jack the Tripper: a spiked chain fighter who trips enemies when attacking, then makes attacks of opportunity when prone enemies try to move or stand up, and steps back 5 feet during every combat round, forcing any approaching enemies to enter his threatened squares again, and get AoO'ed and tripped again.
[Edit: And if we're just talking about spells here, there are tons of spells that instantly put your enemies out of the fight. You put your enemies to sleep or into a trance or turn them into mindslaves or whatever, and then you get free coup de gras attacks or you get to make your enemies fight each other.]
Feat bloat specifically, but content bloat more generally
This happens with every system as it ages. The author wants to sell more books. More spells, more feats, and more equipment all adds up to bloat. And for your wildshaping druid, more enemies means more bloat too, because those are your new wildshapes. I never really thought of that as a bad thing. It's just part of the metagame of building your character and part of the social game where you argue with your DM about what should be included.
Casters end up walking nuke launchers at later levels
I always thought of this as their reward for surviving with 1d4 hit points at first level. Also, if you're playing your caster right, you should be benefiting the whole group as you get XP because you're making magic items for them, thus exploiting the River of XP: When you're lower level than your partymates, you get more XP in every encounter. By staying just one level below them on their way to 20th level, you should accumulate something over 1,000,000 XP more than they get, which you should use to make magic items in order to stay below their level. This brings me to my next point, which I don't think anyone else has mentioned yet...
Wands
This is how you properly play a walking-arsenal caster. A spellcaster can use a fireball or two during his first encounter of the day, but the bulk of his firepower should come from wands. When you go into the Cave of Infinite Bugbears, your wizard can smoke maybe four or five of them per fireball, and if you just learned that fireball spell, you're not going to use it more than once or twice a day, so that fireball spell is worth up to 10 bugbears' worth of XP per day and then you hang back and cast Magic Missile until that's used up too. But if you learn that fireball spell and then immediately make a wand of it, then you can suddenly cast that fireball spell 20 times in a day (using up the wand in the process), killing about 100 bugbears in a day, and the XP value and treasure value of enemies incinerated via wand is worth way more than the XP and gold pieces you spent making that wand. And then you should take that extra XP (extra extra XP if you're also lower-level than your partymates and thus benefiting from the River of XP) and make another wand with it.