Its the 'other Ranger powers' part that doesnt add up. What part of 2wf and archery powers makes you a better beastmaster?
Well, TWF powers come in quite handy, if you want to delve into it, since they synch up with your ability scores and melee-range focus and Striker role quite nicely. Those also give you broader equipment and feat selection, without hurting your ability to use
Synchronized Strike even a little bit.
But the implication seems to be that cleric powers won't make you a better runepriest?
Well, they do, in the same way TWF helps you be a better beastmaster.
I get that it doesn't key directly into the specific schtick of the build, and I'm not saying there's not room to add support. Clearly, horsemasters and monkeymasters could use some support too!
But you don't see people decrying the lack of horsemaster support as the reason that people don't play horsemasters.
Because the few people that <3 horsemasters can make use of a broad existing pool of abilities that are already there that might not be directly related to horsemastery, but can enhance what they do anyway.
If runepriests were a cleric build instead of their own class, that the support need might be a bit less urgent, or less pointed. In the same way that no one's been clamoring for the necessary support for beastmaster rangers who happen to choose "serpent" as their companion, since those beastmaster rangers do have other options that play to their strengths that they can choose from the bigger pool of Ranger abilities. A runepriest could still make use of some specific build-support, but it might not be as vital, as urgent, or as necessary, since they could then choose from the bigger pool of Cleric abilities (think of them picking up Warpriest or Str-based cleric powers, forex...).
At the expense of being a Runepriest...then you do have a Cleric with some odd buffs. To keep the distinction of the build, there would need to be 'Cleric' powers that build on Runepriest features. At that point why arent you just making another class?
I don't see it as "at the expense of" anything. A runepriest that picks up
Levy of Judgement can still use
Rune of Binding, without any penalties or problems. A runepriest who wanted to take ONLY rune powers could still take ONLY rune powers. Only now, they could ALSO take MORE powers, that could help them a lot, even if they weren't rune powers, even if they needed a little re-fluffing.
Again, that's not to say that even a runepriest build couldn't use a Dragon article with some better feat options more appropriate to its schtick or something.
But that IS to say that if runepriests were a build instead of a class, they'd already be much better off than monkeymaster rangers, and you don't see people decrying the lack of support for monkeymasters, since WotC (cleverly) didn't make the monkeymaster into a unique class with 100 powers of its own.
tl;dr version: why does your runepriest need more support than my monkey? Cuz it's a class instead of a build.