As long as this is allowed to happen, eventually there will be nowhere to go but the company store

Branding and a cut of every transaction.

Buying them up and you will have no options except to pay exorbitant prices that insurance will likely not cover fully or without a major deductible. And, even if professional staff (physicians, veterinarians, dentists) do not agree with the corporation's enforced diagnostic and treatment standard policies, they will have little choice except to quit.

Steward hospitals was formed by Cerberus, who saved several failing hospitals to create that company. Cerberus then exited the investment to a private investment group (different from a PE firm), that ran the company off a cliff after Cerberus had exited. Warren blamed the PE anyway.

1) It is really hard to make money on a company that goes bankrupt. The PE firms actually lost money on Toys R Us as defined by not recovering all of the investment they made into the company.
2) If a big box toy store was such a good idea, they would be everywhere. It is fair to criticize Bain and KKR for investing in the deal in the first place, but they are not the reason the company went under.
3) What would be the incentive of any business owner to invest in their own business if the government outlawed pooled funds from buying or investing in a company?

While I generally agree with her, Senator Warren gets on my nerves with these attacks on PE firms. PE firms are a big net positive for the economy, and for employees. PE firms spread ownership much further down the org chart than family businesses, who generally share almost no ownership with employees. It is worth noting that Warren has never proposed an alternative owner for private businesses.