The sighash type byte is a "great" place to store a few bits of ancillary data when making signatures.  Okay it isn't great, but it is good enough that some misguided users may have been using it and have unbroadcast transactions in cold storage (think sweeps) for UTXOs whose private keys may have been lost.  I don't think that one's hunch that there isn't much risk in disabling these sighashes is good enough to put people funds at risk, especially given the alternative proposal of caching the just-before-the-last-byte sighash midstate that is available.

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Russell O'Connor