OP_RETURN data appears in output scripts. Output scripts are not subject to segwit’s witness discount. Per the block weight limit of segwit, we can have up to 4 MB of witness data, but only up to 1 MB of any other data in blocks. Output scripts are part of the other data.

Adding more output script data to blocks would result in smaller blocks. As OP_RETURN outputs require little computation to validate, OP_RETURN data would not slow down IBD.

Unlikely. Inscriptions / witness stuffing is more economical past about the 100 byte mark due to cost. Witness is 4x discounted while OP_RETURNs are not.