There is a difference between feeling weak at a certain phase of the game, which is healthy, and feeling incapable of making an impact, which isn't. If your champion doesn't feel like they have much to do at a certain stage of the game, e.g. pre-Juggernaut rework
and
, then that seriously devalues the experience of playing that champion, as it takes away their agency for large portions of the game. I never said every champion needs to scale well, and having champions that fall off late-game like
or
might even be a good thing, but at the same time they still need to feel able to contribute, which is the issue with a lot of AD junglers now who are forced to build tanky and not be able to do much past a certain point in the game.
Also, the snowballing model of Brutalizer you mentioned isn't healthy design, and Riot's been moving away from it (it's part of the reason why TBC got reworked and had it taken out of its recipe). Sure, assassins are a bit more snowbally than other classes, and that's fine, but the model of old-school assassins like
,
and
, who can be overbearing early on and absolutely have to snowball in order to be effective, lest they become useless, has consistently failed. By contrast, even though you may disagree with the gameplay of, say,
or
, they are successful assassins because they have means of catching up and contributing to their team even when they're behind, and enough counterplay to be dealt with properly even when ahead.
Also, Runeglaive does provide late-game power that Lich Bane doesn't, primarily though its AoE proc, and regardless of how a champion's mana works out it still provides a unique advantage in mana sustain and prolonged clearing. Even in the late-game, a Runeglaive jungler will be able to use more spells than an AP champion without the item, which is why people keep the item in their inventories even late into the game. By contrast, not only does Warrior provide no unique bonuses, which makes every item with more AD flat-out better in all respects, but its scaling is so poor that it's not even worth it by the time people purchase the enchantment, which is why AD junglers usually don't pick it at all and get Cinderhulk instead.