Cloud is already sucked into a second job for AVALANCHE long before he meets Aeris. If she hadn't been in that church, Cloud would have seen Tifa carriaged into Corneo's mansion and he would have gone in and killed everyone (just like if Corneo doesn't pick Cloud which always pissed me off 'cause it was like, WTF was the point then? I'm fighting my way through anyway), they would have gotten back to Sector 7 and the plate would have come down. Tifa would have been the one sent to get Marlene out of the bar so it only would have been Barret and Cloud fighting Reno. Without Tseng flying up to gloat, the two of them would have had more time to get out and probably could have saved Biggs and Jessie along the way... maybe. Maybe even Wedge. The plate crushing 7 is a major push for the group to seek revenge. Despite their claim to the contrary, they are a terrorist group and would be motivated by emotion. Barret would likely ask either Jessie or Wedge (once recovered) to take Marlene out of Midgar while the others fought their way up the Shinra tower to rip them a new one. They still would have learned about Sephiroth and still would have met Nanaki. They'd simply have no knowledge of Jenova or the Ancients by this point, which would mean nothing... they're no worse off. Nope. Aeris lent nothing to nothing.