Do not expect quick travel or interplanar travel.
Quick travel or access to interplanar travel is not a necessity to a campaign or story. Indeed, if the story is 'Man against Nature' or some thing of the such, quick travel may even hinder realization of the story. I don't think you can call something 'underpowered' on the grounds that it doesn't have the means to change the basic plot or basic assumptions of the world. While the ability to dictate the sort of story you are in may be attractive, it isn't the basis for judging whether or not a class is 'underpowered'.
Your statements seem to indicate that a system as a whole can be 'underpowered' if it doesn't provide the easy options and world shattering power of some other system.
Perhaps you should recommend everyone play Scion?
Most optimizers set the bar higher.
Well, sure, most optimizers would certainly set the bar higher than the reasonable expectations of a tier 5 class. But most optimized builds can take on equal CR or EL challenges without assistance, and to truly strain a party of 4 optimized builds would probably require at least 3-4 EL+4 encounters in a day. In my experience, with optimization, EL = party level encounters require negligible expenditure of resources. This is obviously a different approach with different expectations.
Take your hydra suggestion. We could just as easily look at Fire Giants or Bebelith, but 11 Headed Hydra is a fine example. Suppose we have 4 characters dealing 25 damage each. The our hydra with its 120 hitpoints at fast healing 21, if it doesn't put up a strong resistance, is going to die sometime in the second round.
Against the Hydra, I don't think our Warmage is going to feel useless assuming the rest of the party is built to a similar tier.
The Hydra has a move speed of 20', and a charge of 40'. The Warmage has several options:
1) Mass Fire Shield - If everyone has Fire Shield up, each attack by the hydra inflicts 1d6+10 damage to the hydra. Full attack by the Hydra on a party so protected can quickly kill the Hydra. Of course, optimally, the party avoids this fate but even in the worse case this expedient alone is probably going to win the fight provided that Hydra can be made to stick to a 'tank' whose hit points at level 10 are going to be comparable to the hydras (85 or more) and who has an AC high enough that say 1/3rd or more of the attacks will miss. After that, it is the application of a single heal spell to restore our pounded tank to fighting strength.
2) Evard's Black Tentacles - Tradiational hydra tactics revolve around 'kiting' the hydra so that it is kept at a distance for as long as possible. The Warmage is exceptionally well suited for this as a ranged attacker capable of dealing 20 or more damage at range for many rounds in a row, without resorting to a full attack. Additionally the Warmage has limited battlefield control. A field of tentacles will require the Hydra at least two rounds to cross, during which time the party can adjust and pour on ranged fire. There is also a reaonable change the +18 grapple bonus of the tentacles will beat the hydras +25, sticking the hydra in place for one or more rounds.
3) Multiple sources of ranged area of effect fire damage: The war mage is going to be able to regularly hit the hydra with ranged damage doing an average 35+ damage prior to the save, with about 50% chance of save. Average damage is going to be around 27, sufficient by himself to overcome the fast heal and automaticlly burning any exposed stumps should any heads have been severed.
Presumably the rest of the party will be roughly as capable as the Warmage in this rebalanced game. A CR10 hydra may well be a considerable challenge for the level 10 Tier 5 party, but I don't feel that in that situation a level 10 Warmage is going to feel like his presence is unjustified or that somehow his abilities 'invalidate themselves'.
Again, it's not only a problem of lower tiers not being competitive with higher tier classes, it's that they have difficulty standing up to the challenges they're supposed to take on.
The challenges that a party is supposed to take on are almost completely arbitrary. If from the conception of the game, every class was as powerful as Commoner, then all that would change is we'd have a different scale for what monsters made a balanced challenge at a given level. When we adjust the numbers, all we are adjusting is arbitary numbers. The tendency of the game has been to continually adjust these numbers upward in to higher and higher reaches. But the fact that the numbers are now arbitrarily larger doesn't necessarily change anything. One of the reasons I didn't enjoy Diablo II (for example) is that the math was so fixed that the game played pretty much the same regardless of whether I had 20 hit points or 200 or 2000, or whether I was doing 10 damage or 100.
