One of the mysteries that most Americans fail to grasp about Germany, and the immigration issues....is the lofty status of Chancellor Merkel, and how public criticism remains at low-throttle. So I'll lay out the four basic reasons.
1. No scandals. You can go to various past Chancellors and find political or personal scandals brewing. With Merkel, none. Ethics wise, she pretty much maxes out. She's aware of that and is kinda proud that her path is fairly clear. You can go back thirty years, and find virtually nothing to write some scandal piece upon. She might sip an occasional wine or beer. She has a particular list of hobbies or passions....none really drawing any attention. Most of her public statements are carefully crafted. In the simplest of words.....she's the professor-type but along the lines of a fairly boring professor with a clear path.
2. She doesn't insult anyone. You see a lot of US and Brit political figures who openly insult opposition leaders and occasionally members of their own political parties. She's fairly careful about insults. You can't find even a harsh word commented on Putin. On public speeches, she might utter some disappointments about people....but it never fits into the insult department.
3. If you dumped Merkel, then who? This is one of the great problems of the German political system. You can go through the whole next ten folks of the CDU Party (her party) and find that none will get public attention or draw massive appeal throughout the public. I'm betting various polling instruments have been done by the party and readily prove that fact....hence the reason why she runs yet again. From the opposition parties? The SPD has a list of three possible people who will be the ultimate candidate for them....but none of the three push their numbers up by more than one or two percent. The Greens have four characters in the running....same issue. Linke Party....same issue. It is a one-horse leadership situation, and the public has no problem with that.
4. The general news media....from state-run/public-TV, to the commercial TV side....and onto print media....have either led with neutral stories or positive stories of the Merkel period. Some print-media sources have done criticism pieces of the immigration business...but never really connecting Merkel to the blame side of this. I would agree...the pro-immigration tone set by state-run/public-TV has soften over the past two years....but no one does a blame-game situation (like you'd see in the US).
At present, I'd expect the election to conclude in late September with the CDU/CSU combination getting a minimum of 30-percent (maybe even 36-percent as it stands today), and things continue on. It would take a shocker to see any public shift and result in lesser numbers. The second place crowd? Presently the SPD, with 20-percent.