Well, the question is, is SPD ready for a red-red-green coalition? (or red-red if the greens fall below 5%)
If the answer is yes, then I agree that by 2021 they could regain the chancellorship. Merkel will allegedly retire and she has no clear successor. A good and charismatic SPD candidate could push SPD above 30%. At that point red-red-green probably has a majority.
If they are not willing to go with red-red-(green) then I seriously doubt it. Eventually the left will win an election, and unlike here I don't see Linke becoming the number 1 choice for the German left. But it will probably be later than 2025.
I guess they could go with SPD-Greens-FDP but both the Greens and the FDP will fall after Jamaica.
I think they are ready if the Left moderates on foreign policy. Especially if the pragmatic eastern-wing takes control. However, I'm sure that a coalition at the federal level will hurt the Left because they have to make compromises. Especially such compromises their base won't like much. Maybe that's chance to make the party irrelevant in the future (keep also in mind that the Left has one of the highest age-average of their members).