Companies claim mergers of competitors are good because they increase efficiency, to get permission to centralize markets to a far greater degree than any cartel could.

Once under more centralized control, new and old efficiencies are moved from customer benefits per charge, to conglomerate revenue per expense.

The centralization enables the change, and defends it from competitive pressure.

And regulators keep falling for it, because industry money has so many ways to push watchdog decisions in the direction they want, under the cover of relentless PR.

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It would be a very blunt instrument to require companies that reached 50% market share, or $500m valuation, for more than three years to split into independent companies. In any way they wanted to organizationally and asset-wise, as long as the highest valued component was valued at less than 60% of the original. (Strategically owner/leadership designed breakups often result in a greater sum value. So more than one component may end up worth more than 50% of the original.)

A very very blunt economic instrument, indeed.

But I really think markets would become more dynamic, competition fiercer, technological growth faster, economic growth higher, and customer benefits greater.

Great for the labor market too. Both in job creation and economic mobility. The continual emphasis on developing new leadership talent for success created spinoffs would be significant.

Startups would have fiercer competition in terms of incumbent adaptation and innovation, but lower passive barriers based on scale, brand, etc.

Billionaires would continue to be minted. Warren Buffet adds value to many companies without creating self-serving keiretsu out of them. Other billionaires would tilt more toward the multi-founding pattern, instead of the single-company (or tree of controlled subsidiaries) mogul type.

(I am aware that some markets, especially some utility type markets, "want" to be monopolies due to objectively high costs of duplication. But even those can be made more decentralized and more competitive by increased modularity on functional lines, and similar decompositions, suited to specific economics and practicalities.)