There is no problem, energy is "free", but still, the source of the electricity should always be a concern not just for bitcoin but for everything. Nuclear, solar, wind? good. Hydro, natural gas? less good but ok for now. Carbon, oil? we need to replace those plants asap.

Bitcoin or anything really should be allowed to use however much energy it is economically feasible. The real question is, why is there no better use for that electricity than to compute numbers? How our economies became so stagnant the best return for capital investment is bitcoin?