Late game isn't determined by infinite scaling, it's determined by reliability of damage output, peeling, and utility. You say Nasus is late game but that isn't true since season 3-4. All he can do late game is split push and that is not as good as team fighting late game since a 5 man team can shred a 4 man team late game in a blink of an eye under the fountain. If Nasus does team fight at any point of the game he gets kited and exploded. The only late game infinite stacker you have there is Viegar because of his aoe utility stun, massive damage, and a point and click ult to nullify a squishy or a mage or both.
Sion is a CC punching bag that requires CC help to land his CC. Thresh and Bard are support made to make plays, not end them. Kindred drops late game because of her auto attack range being dangerously short. What good is an aoe prevent death if being stunned near the front line for 1 second is death for nearly everyone, including a tank, come late game?
Late game is also determing by items too. No other type of items synchronize as good as ad items. Every ad item boosts a marksman damage output by multiple instead of additive. Offensive items are multiplied and defensive items are additive. This is to prevent games for going on forever.
Other reason some marksman are late gods are because of their kits:
Vayne has unpreventable damage if landed right and peeled.
Jinx has long range safe and extremely damaging aoe rockets that crit on top of that.
Tristana has supreme self peel and long auto attack range, combined is great reliable damage.
Kog'maw is a long range machine gun that requires the right group to peel for just a moment.
Jhin has long range massive damage between his auto attacks that speed boost him and his ult.
Sivir is considered late game all just due to her ult and wave clearing.
Ezreal because of being blue on top of his kiting capabilities with poke.
So there you have it, infinite scaling doesn't determine late game.
Edit: Mobility on top of a good kit makes or breaks late game as well.