Why did Taproot retain a 520-byte push limit?

It seems that the way you think about it is backwards. You need to claim it is safe remove DOS limit. It must be in the default position to hold it. Especially as other restrictions have been lifted.

For example, if there is no stack element size limit, you can use the following tap script to create a script that uses ~4GB of memory to verify:

<396000 bytes push> OP_3DUP OP_3DUP .. OP_3DUP

At 333 OP_3DUPs This creates a stack with 1000 3.96MB elements and uses about 3.96GB of memory to store them. Of course, there’s more you can do, but the memory explosion is probably the most obvious example.