Even while prunable, they have to be processed in the mempool and in the initial blockchain sync, which can be a huge bottleneck for people with less computer resources. I agree with you on the segwit 4mb blocks, it’s too big. Just some MBs can make a bitcoin node too expensive to run in places where it’s needed the most, just like arbitrary data can use memory and cpu power that is wasted, knowing it could have been used for monetary transactions, even if ignored after. If it can be pruned, it could not be put there in the first place.