Or are you talking about SegWit blocks stored by a Legacy Node?

If you are asking about SegWit blocks stored by a Legacy Node, then those nodes are fooled into thinking that bitcoins sent to a SegWit address are sent to a "ANYONE CAN SPEND" output.  Therefore, when they try to validate the blocks that spend those bitcoins, they are unable to tell the difference between a block where those bitcoins are validly spent or a block where they are invalidly spent.  In other words, a Legacy Node is unable to fully validate a block that has transactions spending bitcoins that were previously received at a SegWit address. Instead, it must trust that those blocks have been validated by a SegWit node.

If you are asking about SegWit blocks stored by a Legacy Node, then I've already explained that the Legacy Node can't tell the difference between a valid transaction that spends bitcoins which were previously received at a SegWit address and an invalid transaction that spends bitcoins which were previously received at a SegWit address. The Legacy Node must trust that the block has already been validated by a SegWit node.

Except that we would miss out on all the benefits that SegWit provides.  SegWit was not intended to be a method of significantly increasing throughput. It was intended to fix some issues with bitcoin transactions (such as maleability), and to make other transaction types easier or possible.