The block has to be accepted by all nodes (consensus). Proof-of-work is one such consensus scheme: nodes on the Bitcoin network verify that block hashes are valid and meet the required difficulty. You're suggesting proof-of-bottom-50% for the altcoin reward but there is no viable verification method for it.

E.g. if Antpool mines a block Coinbase has to accept it otherwise you wouldn't be able to transact those mined coins with Coinbase.