Maintain credibility with whom? Do you think the NDP is blowing their credibility with people who would actually consider voting NDP? You probably never will. I probably never will. So do they care much what you or I think?
The NDP, through supply and confidence, has continued to advance their policy goals farther than a third place opposition almost ever can. They’re a party of perpetual opposition who can point to the LPC pushing pharmacare, dental care, etc essentially because the NDP forces them to in order to stay in government. No matter how hard the LPC try to outflank the NDP on the left, NDP can just shrug, point, and say “they needed us so we got this”.
The game plays differently when you have a shot at forming government versus when you don’t. You’re suggesting the NDP lose credibility by propping up the government, so presumably you mean they lose credibility among those who oppose the government… Did NDP have any credibility there to begin with? Or are they just pissing off voters they would never get anyway? I don’t put much stock in polls, but their popular vote is polling not far off what they got in the last election. That’s not suggestive of any significant credibility loss. I don’t think people believe or respect them generally less than they did before.