BIP42 is not where the 21 million supply comes from. If you read any document about Bitcoin's early history the 21 million limit is almost universally mentioned. I guess there are many people these days that didn't know Bitcoin before 2014, but it wasn't that long ago. It seems completely unrealistic to think the Bitcoin ecosystem would ever have accepted such a dramatic change in its economic policy. Having the policy being fixed in stone and beyond influence of a central bank - or anyone - is one of Bitcoin's primary selling points.

Furthermore, BIP42 doesn't even claim that it is the origin of the 21 million limit. It claims (jokingly) that Bitcoin was designed to look like it had a 21 million limit, but through underhanded programming didn't actually do that.

The reality is that ever since the January 2009 release of the software, the 21 million limit was there, coded in by Satoshi. In March 2014, "ditto-b" on Github noticed a bug in the implementation in Bitcoin Core (and not in other software), that would result to start another cycle of 21 million bitcoins to be issued 250 years in the future. We took advantage of that situation to write an April's Fools BIP that proposed his fix as an actual consensus rule change (which was correct), but also incorrectly misrepresented reality as that bug being intentional.