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Myth: bitcoin does not have the drawback of fiat money - the total BTC number is 21 million.
The developers could change the codes tomorrow to allow 1000 million BTC in 2140
I assume that by "developers" you mean "Bitcoin Core developers", because it's important to note that a significant number of people run clients other than the reference client.
However, this is entirely false. If the Bitcoin Core client was altered in this way, people would have to choose to upgrade their client. If people still run clients which involve having a max of 21 million coins and they continue the chain with a max of 21 million coins, there will still be a max of 21 million coins.
But now, we have these "open source" central bankers deciding
on the money supply.
False. Central bankers decide on the money supply, whereas what Bitcoin Core developers do (not that they have any intention of doing this), is ask people to support an increased money supply. People could still choose to support the previous money supply and it would still exist.
"Theoretically yes, if all the miners and payment providers adopted a new system which allowed for more BTC to be issued to miners in future it can but would make a hard fork.
Due to a hard fork needed in practice it is so incredibly unlikely to happen I'd say not, at least not in our lifetime. The issue is if more were to be added in would shake confidence in it being a finite resource so anyone involved in the ecosystem would lose out.
Look at how difficult it is just to increase the block size from the temporary 1Mb to 20Mb being proposed... if bitcoin is still around in 10 years it's going to be even harder to make a significant change like that.
Who knows though, it's hard enough to see 5 years into the future never mind 50."