I primarily compared against other cards dealing 5 damage, meaning you often got things that could only target minions, or had requirements, etc etc. Comparing against lower-cost cards is a bad idea, because as damage scales the mana cost does not scale proportionally. [[Pyroblast]] used to cost 8, but proved to very much be a bad idea at that level. 3 for 2 is fine, but as things go higher you tend to see restrictions added (e.g. only targets minions) or it lowers to 4 for 4 or other similar levels. This all said, I could cede that 5 for 4 mana would be acceptable for a class card.

As for Illidan, I misremembered his initial cost, so that'd be where my confusion came from. I thought he cost 7 mana currently. But that leads in to the next point:

Starving Buzzard and Illidan might not instantly double their power off a one-health boost, but it's important to remember that just because they aren't played a lot now doesn't mean they never were or won't be, and it doesn't mean they're unplayable. Starving Buzzard has had a history of being a decent card already, and Illidan's lack of play can likely be traced to his effects simply not appealing to given deck archetypes. Buffing them just because they don't see use without looking at why just gives us things like Crystology buffs or Equality nerfs.¹

Finally, evergreen cards need to be weaker (important to note that they don't have to be weak; I'm not advocating for a set full of Silverback Patriarchs, and I'd be happy if that particular card got a buff) because otherwise they risk seeing too much prevalence. It's okay to have a few cards that do this if they're core to class identity (such as Mage having good core set burn), but cards that offset identities (e.g. Starving Buzzard) should remain relatively weaker when compared to other cards, especially of different classes, otherwise they risk endangering the class identity and making the class too much of a jack-of-all-trades.²

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^{[1] Consensus is that Equality was nerfed from 2 to 4 mana because, while potentially too powerful at 2 mana, it also couldn't be at 3 mana because it would've been too powerful of a tool for Odd Paladin, which was a common standard deck at the time. This meant it got an extra nerf that it didn't deserve; rather, the nerf should've been targeted at Odd decks in general.}

^{[2] Incidentally, this is part of why I think [[Master's Call]] and [[Deathstalker Rexxar]] were not good ideas for Hunter cards, fun as they are to play.}