Yes, Federer's comeback will last because he has proven that he is one of the best players in the world. He has dedicated his time and money to making this comeback possible. He will not let distractions get in the way of victories, and all fans should support him going forward.
If Roger Federer is able to focus on the task at hand, there is no reason his comeback can't be a long lasting one. Everyone has ups and downs in their life. It is possible he was going through one of those down times. Distractions and personal events in a persons' life can cause them to temporarily lose focus.
Roger Federer will never be as good as he was during his prime. He is 35 years old now, and he is still one of the top players in the world, which is an amazing accomplishment. However, no one can beat time. It's great that he's still competing at a high level, but it won't last much longer.
There are very few comebacks that are actually successful. Shannon Miller failed to make the 2000 women's Olympic gymnastics team. Tiger Woods has never successful comeback despite years of trying. It's too hard to stay at the top of any level of sports. Once it's gone, it's gone, and that's the case in Federer's case.