Agreed, I don't think it was a lucky punch, either, but I've seen enough MMA to know it wasn't a tell-all type of fight. McGregor had a more difficult fight against both Mendes and Siver than he did Aldo, and by itself, tells us that in the 13 seconds it took, the night was his-- but it wasn't indicative of either mans [Aldo and McGregor's] full potential.
I've seen Rockhold get finished by Belfort, only to go forward and defeat Weidman. I've seen Cain get finished early by JDS, only to absolutely destroy him in two more contests, afterwards. McGregor certainly defeated Aldo impressively, but the most it taught us was that McGregor can take out anyone at FW with his power. Even McGregor stated during the conference, that he would've liked it to go a bit longer, and I think most reasonable fans would agree.
Realistically, they probably weren't. A fight camp is designed to prepare a fighter for peak physical/mental efficiency and it requires time, money and resources, not just for the fighter, but for the team they must assemble. You're focusing on Mendes, but you also have to consider the group of people behind him who contribute to his success.
While Aldo may have pulled out of fights previously, the decision to continue with the event was also on McGregor. If he instead decided to wait for Aldo, the UFC would have likely obliged and postponed the event. McGregor decided to go forward with a fight (bold of him), and that meant the UFC was now forced to find a willing opponent. It doesn't mean that opponent could have realistically anticipated this.
On top of this, is the fact that neither Mendes or Edgar could be 100% positive that THEY would be the backup guy, which would result in spending a lot of time, money and resources on a very uncertain gamble, especially when the injury occurred that close to the fight.
At the end of the day, both McGregor and Mendes signed the dotted line, so credit to the victor, but the devil is in the details.