Most of the people who put spam in utxos use a protocol called Stamps. The Stamps protocol was created out of an explicit desire to put the data in the utxo set. See, for example, this quote from the cofounder: "The point of Stamps is to offer a method of encoding on-chain art that is highly resilient to tampering and goes a step further than Ordinals by being unprunable." source: https://thedefiant.io/news/nfts-and-web3/bitcoin-stamps-seek-to-improve-on-ordinals
Enlarging op_return to accommodate these people and make their data prunable seems pointless, as they have explicitly said they don't want their data to *be* prunable. If the point is to accommodate other, "less popular" protocols or one-off spammers, then the cure seems far worse than the disease: apart from Stamps, hardly anything uses output stuffing, so even if those other unpopular protocols *do* switch to op_return (which I doubt many will do), you'll get a very small benefit in that one respect at the expense of making low-effort spamming far easier for the masses.