The reason it [attempting to address data spam at the relay level] is harmful for the ecosystem at large is mining centralization. Given enough demand for transactions that the network of nodes shuns (for whatever reason), large miners are incentivized to develop means for users and companies to submit these shunned transactions directly to them (this is already happening to some extent). If these "out-of-band translation relay rails" end up becoming economically relevant enough (through the out-of-band fees they involve) that miners not using them become uncompetitive, the ecosystem has a huge problem, far bigger than some temporary JPEG hype drivel: the inability for new small miners to enter the ecosystem (because who would bother sending transactions to a tiny miner), especially anonymously (the best protection the ecosystem has long term against miner censorship). In addition, it incentives the development of one or a few centralized "payment submission network" companies that all big transaction creators and miners contract with (something we've seen in other blockchains), imposing a large censorship risk on this company, and making the entire goal of decentralization a shim of what it was supposed to be.