Not government, not Elon Musk, not Blackrock, etc are the Bitcoin competitors.
The biggest Bitcoin competitor is centralized exchange, they're creating a fake system that harm the decentralization. Unfortunately most people don't aware with this and they still use CEX to exchange or hold their coins. As long as people still use centralized exchanges, we haven't win against centralization.
what stops them from building a coin on the block chain that'll bypass the total sum of bitcoin users, and regulate it?
I think what you are referring in this one is the geographical and political risks involve in the widespread use of cryptocurrency. The government of a certain country may push multiple policies to discourage their citizens to use bitcoin. Or maybe they will provide policies that will encourage their citizens to choose their central bank instead of choosing a decentralized technology.
There are already more than 20K altcoins exist until now, is there any coin ever overtake Bitcoin? nope, it's proving anyone who create a new coin can't beat Bitcoin.
My hypothesis is maybe a coin/chain that can host other altcoins (e.g. zetachain, eth, wax) to fulfill a specific purpose might rival Bitcoin. The selling factor of some altcoins is that they want to use decentralized technology to solve specific real world problems (a master of one than a a jack of all trades)