They don't take up the "same space". In order to validate blocks utxo data needs to be quickly accessible as the access speed bottlenecks validation speed, witness data doesn't need to be stored *at all*, since once you've validated it once you can forget it, so the long term cost of witness data is orders of magnitude lower.
i'm not sure what you mean by "utxo data" but utxos don't have signatures attached to them. they have to be signed to be spent. a utxo is just a transaction id with a particular output index number. nothing more than that. in practice though i think the output index number is looked up and replaced with the value and scriptpubkey. but still. we aren't needing to store any signatures in the utxo set.
for non-segwit transactions the signature is stored in the scriptsig field of transaction inputs. but they are not needed. so once you have validated a particular transaction you could just throw it away. and just store any of its outputs you needed to in the utxo set.
In any case, the perspective you're adopting is a confused one-- I think a confused one specifically engineered by malicious parties attempting to engage in consensus cracking.
i understand that segwit solved transaction malleability and that it segregated the signature from the transaction itself. those things should have been done in the very beginning but anyway. i'm not arguing against the merits of segwit but you are/were (?) a bitcoin developer. surely you can't really agree with allowing a segwit signature to hog up the entire block taking up the full 4MB can you? what do you think the developers were thinking when they allowed that loophole?
There is exactly one metric that matters when it comes to the ability to spam and the cost of spam: the capacity of the network relative to the demand. Segwit did increase people's ability to add spam, but it did so purely by virtue of adding capacity. Any other way of adding capacity would have the same effect.
keeping non-segwit block size limit at 1MB but telling people "oh, you can do segwit transactions and the block can go up to 4MB and not only that but you get a 75% discount on the fee and guess what, you can store arbitrary data too in the witness signature and it can be as large as you want it to be" that's about as open an invitation to spammers as you could make. kind of unreasonable!
Perhaps the particular spam *encoding* chosen might be different depending on how the capacity limit was constructed, but that's immaterial to anything you care about. (and fortunately, the weighing scheme has caused the spammers here to encode their bullshit as witness data, which radically lowers the carrying cost of it for the network).
you're acting like they will always find a loophole. well it wasn't very hard to find with taproot was it? it was more like an open invitation to spam the blockchain with huge data.