The incredible douchbaggery of Cloud and Friends

[Sephiroth]

1. Using Materia's power harms the planet as well. As in, greatly. That is also why they do not use it again in Advent Children (granted - if one day they decide to make an FFVII-2 they surely for out-universe reasons bring them back like they did in DoC but I am just stating what in-universe reasons for charas are). Hell, before they got to know Hojo wanted to help Sephiroth they also tried to stop him as they did not want to harm the planet by letting the Sister Ray shoot at the Northern Cave once more.
2. It was not just that Rufus' plan failed because they interfered. It was Sephiroth's mind which held the summoned Meteor together. As you might have noticed, the Meteor WAS blown to pieces. So Huge Materia or not, they would have failed. Sephiroth controlled the summoned Meteor and he held back the summon of Holy. There was nothing they could do except to defeat Sephiroth. And even that was done too late as Meteor was too close at that point.
3. Why would they have "no plan"? Cloud's entire journey was about confronting Sephiroth to settle things. Of course his main goal was to face his past and fight it but it would still be connected to Meteor. The entire point was that they all had a reason to fight for. And to fighting for what they loved they would have had to face Sephiroth and therefore the reason of Meteor anyway. They did not say "we are going to fight him" all 5 minutes but it was obvious that there would be no getting around that. And Cloud totally new that when right before CD2 ended, they had their talk about what to do next. First they relied on Holy. But what would have happened even with Holy working? Would they let Sephiroth stay in the crater and wait for him to summon the next Ultimate Destructive Magic? No. Sephiroth always was the root to be destroyed. They all were very well aware that there was a guy meditating under their feet and that maybe not now, maybe not tomorrow but one day there would have to be something that they had to do against him. Especially as over the course of those years he already in fact did absorb much of the lifestream's energy already (with his mind travelling through it, e.g.) and totally without a summoned Meteor. Eventually they would be running out of time.