As much as it's easy to forget, the free market is supposed to be a tool for the benefit of consumers (through both price and innovation), and while it's not very common, every once in a while the government actually looks at that as important and steps in.
The reason for competition law is to regulate the free market to ensure that consumer choice is not limited by the uncontrolled acquisition and merger of suppliers.
The merger of NVidia and ARM is inherently non-competitive unless NVidia accepted limitations/restrictions on their licensing freedom which they are not willing to propose or accept.
That's leaving aside the international and other ramifications of a company that is involved in the entire vertical of chip production and sale through to retail having control of the licensing of the most used ISA and other IPRs that are available to its competitors throughout that vertical.
This isn't a question of "will the economy collapse if X buys Y" it's a question of "Which will yeild the best outcome, Y being acquired by X or no". The exact definition of "best outcome" is where the entire battleground though.
It is quite difficult to see who actually could acquire ARM though - because of ARM's quite unique position, any of the potential acquirers are either customers (Apple, Nvidia, Qualcomm) who would be vertically integrated to beat out competitors or direct competitors (AMD, Intel). I think almost the only people who could buy ARM at this point would be TSMC.
Softbank can still sell ARM to anyone including the public market, they just won't get as much money for them that way. Presumably.