It sounds a bit implausible. I wonder if it is possible to preserve energy over very long periods to later use it more effectively. We are talking billions of years.

I think that the 'suicide' explaination is more plausible. When an advanced civilisation concludes that there is no eternal life, that there is no ultimate meaning to life and that life always comes with suffering, it might decided that the best solution is not to produce more offspring or commit active suicide.