Most fighters won't be in 5-round fight preparation at all times. Some have actual day jobs, they need time with friends and family, or generally time to recuperate from little injuries etc. Many fighters will stay in good, non-fight-ready shape, but as you've hinted at, that won't compare to another elite level fighter who has enjoyed a full camp (before factoring anything else in). We saw where the change in opponent disrupted McGregor, and that could be a topic unto itself; here, we saw where the short notice hurt Mendes. At the end of the day, it is what it is.
If the UFC really wants to address the problem of injuries disrupting big fights, they can consider a contingency plan where they pay a fighter their show money to train as a backup, just in case they need to call them. But without some type of guaranteed pay or something for their training efforts (should they
not be called upon), it makes more sense to do what Mendes did and just fight on short notice.
As to Aldo's track record, he still generally maintained a schedule of two fights per year. There is too much uncertainty involved to really dedicate the time and resources to a full fight camp, based on this.
It can cost more than you think. You know the fighter pay topic? This is where it comes into play. A number of them do this part-time and still balance it with a day job.
Here is one article detailing the cost for entry level fighters, pegged at $4,000-$12,000 out of pocket (you should read the entire thing, actually). Figures for a full, "elite" camp can be found,
here, and seem to be as high as $127,000. Even if we wanted to be conservative for the sake of discussion and go with 1/3 the number, that's still around 40k...and fighters like Cerrone have confirmed that it's definitely higher than that. You'd owe people a lot of money if you're a backup and you DON'T get called to fight.
Above you said that guys who don't commit to these camps aren't championship material, but the thing is, most
can't afford to. A guy like McGregor or some of the other top guys who make good money have the resources, but for everyone else? It's expensive and risky. To keep it simple, if it were as easy as just watching tape and performing, fighters would be doing it more often.
Overall, keep this stuff in perspective-- at the end of the day, McGregor won the fight. We can discuss details all day long, but if McGregor is as good as he says he is, he'll continue to win fights until he removes most doubt (after which, he'll still have detractors...that's just how it is). Watching fighters consistently prove themselves is part of the fun in watching the sport.
