That’s because there’s more and more passing so your linebackers are spending more time in coverage and less time trying to shed blocks, plug gaps or tackle 240 pound running backs, which is different than what this conversation is about. It’s not easy to simply “avoid the block” when you’re trying to tackle in a tight space simply by being faster, which is why linebackers still play. A 220 pound safety trying to avoid getting blocked by Gronk is going to end up getting pancaked.
Everyone is in agreement though that the tackle machine linebacker with mediocre athleticism (Zac Thomas, Takeo es, etc) is a thing of the past, but you still need to have some size at the linebacker position for between the tackle running.