EIP-155 support by a particular chain doesn’t imply any sort of compatibility other than signing mechanism for a subset of transactions. Even now, there are Ethereum spin-off chains that have relatively few EIP-155 transactions submitted, and Ethereum Mainnet has an ever dwindling number of eip-155 transaction signatures.
Using eip-155 as the prefix feels similar to using any other EIP number as the prefix like EIP-2718 or EIP-55. You can very easily (and we may already at this point) have two Ethereum-like chains that have a chain-id but one of them doesn’t support EIP-155 transactions at all (in fact, there was talk just today about deprecating EIP-155 transactions in Ethereum).
I think the core of my argument here is that the eip155 prefix doesn’t provide any additional value to the user or developer that ethereum wouldn’t similarly provide. I still argue that we should have a prefix per chain rather than trying to draw arbitrary lines around chain groups because that gives you real actionable information, but even if we ignore that and try to do a bucket-prefix, EIP-155 doesn’t tell the reader anything useful that a random number or word would.