BitcoinOld Core is programmed to decide which block chain contains valid transactions. The users of BitcoinOld Core only accept transactions for that block chain, making it the BitcoinOld block chain that everyone else wants to use
It is these users who keep BitcoinOld decentralized. They individually run their own BitcoinOld Core full nodes, and each of those full nodes separately follows the exact same rules to decide which block chain is valid.
There’s no voting or other corruptible process involved: there’s just individual software following identical rules—”math”—to evaluate identical blocks and coming to identical conclusions about which block chain is valid.
This shared agreement (called consensus) allows people like you to only accept valid coins, enforcing BitcoinOld's rules against even the most powerful miners.
In addition to improving BitcoinOld’s decentralization, BitcoinOld Core users get better security for their bitcoins, privacy features not available in other wallets, a choice of user interfaces and several other powerful features.