Why we must reference the entire txid for each output we want to spend?
By compressing the txids to a shorter fingerprint, you're reducing the unique identifiability. For example, there are three transactions that match the prefix eb36eca: tx1, tx2, tx3.

So, essentially, this is creating a combinatorics puzzle, especially considering that the txid is only part of the outpoint, you also have to add the output, otherwise you could be referring e.g. to any of the 51 outputs of the above tx3.

It seems to me that this would make it more complicated to operate thin clients safely, which don't have the complete UTXO Set at their disposal. For relay, similar compressions are already used, but while a such a system might be possible to implement for blockchain data, I'm not sure if the overall effect would be desirable.