I looked at this again. It's odd they're paying a dividend but have all that debt.
I think this is one where they will VERY slowly delever over time, and will probably survive. I don't know how shareholders do very well here though. ATP stated that they didn't receive any bids higher than $3.04/share which makes the stock quite unexciting, unless you think they received bids for just less than that.
But my guess is the activist board seats mean that they truly can't sell the company for a desirable price now and so the activists want to make sure the debt gets paid down to recover some of their equity. I think FCF is something like 2-3% of total debt.
I should note that the senior notes could be attractive at some point if there's distress in the sector (they trade at par now) - as they're the next major maturity and rank higher than most or all of the debt (haven't spent tons of time on the cap structure but know that many of the issues are junior).
But wow, they have some nice assets: gas, wind, and nuclear generation capability. Berkshire buys some power from them and I'm sure evaluated buying the ATP.