Three things:
1. I doubt that this would make them a sub 20% party permanently
2. Just take a look at green policies before they went into a coalition with SPD and afterwards. They had very extreme positions on foreign policies and all of them were swept away right away. It was not a major focus of their policies and an easy thing to give up. Same will happen if the left enter a government
3. In general, you are acting like the left would suddenly decide everything on their own if they enter a coalition. I claim it would be disastrous for the left if they insisted on their crude foreign policies to the point of rendering improvements in social justice that would only be possible under a red-red-green coalition, impossible. As a small party, the left has a core topic, which is social justice in economical terms. If major advancements in this central topic are made (Hartz 4, tax distribution, social insurances), a coalition they are part of would be successful in the eyes of their voters, even if NATO is not even brought up in the slightest. Similar to how the red-green coalition was a success to green voters due to major advancements in green energies and nuclear power.
Regarding the tax issues, yes this is indeed a bit problematic, because people continue to easily fall for claims that something to their personal disadvantage may happen, when in reality, only very rich people would be affected negatively. Case in point the tax concept of Trittin last election, which was only "expensive" to people who earn a shitton of money. However, due to inflation, the middle incomes are negatively affected by time, so a reform is necessary and I indeed think that a taxing closer to the 1997 level for highest income levels is fair and the right way to go.
