It's a legitimate concern. After Standard Oil was split up, Rockefeller still arranged meetings of the heads of the now-separated companies.

However as long as the companies are well regulated and publicly traded, I think the concern diminishes over time. Each manager has an incentive to look out for their own shareholders, their own bonuses. And if activist shareholders suspect one company is subsidizing another, they can happily buy the undervalued company, raise a ruckus (and maybe some lawsuits), and profit when the subsidies end.

I don’t know about “help”, but it will still change something - over time the different management teams will diverge (in some sense), even if the shareholders stay mostly the same.

Per Das Wiki [1]: "The government had little choice but to return service to the commercial airlines, but did so with several new conditions. The Air Mail Act of June 12, 1934, drafted at the height of the crisis by Black (and known as the "Black-McKellar bill"), restored competitive bidding, closely regulated airmail labor operations,[n 28] dissolved the holding companies that brought together airlines and aircraft manufacturers, and prevented companies that held the old contracts from obtaining new ones."

[1] https://en.wikipedia.org/wiki/Air_Mail_scandal#Effects_on_th...