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Allowing the energy companies to fail is the cheapest way of renationalising the entire system (without buying out the energy companies) so it can be run as an essential service at cost
You have to be a bit more specific when you say "energy companies". My point was about the pure suppliers (Bulb, Octopus etc) that don't actually own any power stations. These are not valuable companies (have barely turned a profit in the last decade) and are of minimal value for the government to own.
The owners of power stations are at zero risk of failure... you can always turn it off if you aren't allowed to sell energy above your marginal cost to produce.
What is the use in that? A bankrupt energy supplier (e.g. Bulb) is of zero value, probably deeply negative if you honour long-term fixed tariffs properly. It's not like you would transfer any productive generation assets to the State.