Remember that the planet is already dying without Sephiroth's involvement. Granted that Meteor is the more imminent threat, but I feel like people too critical of what Cloud's team does there are maybe not taking so seriously the threat that Shinra's Mako consumption poses. My take on it is that they think it probably won't work and that, even if it does, the loss of that much crystallized Mako energy will be too devastating for the planet. On the other hand, Shinra execs really don't take seriously the threat their actions pose, so it makes sense they would try to tackle the problem with a powerful weapon at their disposal. So I don't think it's terrible from a plot standpoint. However, on a gameplay level it begins to feel a little too much like fetch quests.