Doesn’t widespread use of uncapped OP_RETURN risk significant long-term bloat
No, the limiting factor in blockchain growth is the block weight limit, and the interval between blocks. Thiese limit is a consensus rule and remains unchanged – blocks cannot be more than 4 million weight, which equates to a maximum size of 4 MB, and the difficulty adjustment makes blocks every 10 minutes on average.
Furthermore, OP_RETURNs are outputs which cost 4x in weight than inputs, so blocks that contain predominantly OP_RETURNs are smaller in actual bytes than blocks containing mostly input data (as in spending UTXOs, or inscriptions). So such blocks are really 1 MB in size, which means that the blockchain would grow at a slower rate.
(An interesting thing about the blockchain growth rate is that it can actually be larger by a significant margin due to the timewarp atack. Timewarp makes it possible to reduce the difficulty such that the time between blocks can be 6 blocks per second. IMO concerns about blockchain growth rate should be directed towards fixing the timewarp attack, which can be achieved with the Consensus Cleanup soft fork specified in BIP 54.)
slowing Initial Block Download
No. As mentioned above, blocks containing predominantly OP_RETURNs have a smaller size, so IBD of such blocks is actually faster. There would generally be less data to download in such blocks. In terms of validation costs, outputs are not validated in any capacity, so whether an output is or isn’t an OP_RETURN makes no difference. What does make a difference is that because OP_RETURNs are provably unspendable, those outputs do not need to be added to the UTXO set. This means that there is actually less disk I/O and memory usage required for blocks with many OP_RETURNs since those outputs are simply skipped when updating the UTXO set.
raising archival node costs, and gradually pushing full validation into datacenter hardware? In that sense, how is this materially different from the blocksize increases that were previously rejected for centralization concerns?
As discussed above, costs do not increase, and would actually decrease.
