If you had an RTX 2070 fail, then it should still be covered by warranty, right? If that's the case, why not just get that replaced?
As for performance, a 1080 will generally perform roughly similar to a 2060, but lacks hardware raytracing support and some other new hardware features that may offer better performance and/or visuals in future games. So, €400 for a used 1080 with limited warranty coverage seems questionable to me. I don't know what country you are in, but it might be possible to get a brand new 2060 with better warranty coverage for around €350. Even at the exact same price with both cards being new, I would go for a 2060 over a 1080. Of course, either option would be about 20% slower than a 2070.
Likewise, a 1080 Ti generally performs a little faster than a 2070, but again, lacks those new hardware features. A 2070 SUPER will typically perform roughly similar to a 1080 Ti though, and you may be able to find one of those new for about the same price as that used 1080 Ti. So again, at a similar price, I would go with a 2070 SUPER over a 1080 Ti, even if both cards were new.
If you had a 2070 fail, that doesn't mean all 20-series card will fail, and I would be more concerned about how a used card was treated, and what kind of warranty coverage it might have in this price range.
Another card to consider might be the Radeon RX 5700 XT, which generally performs slightly faster than a 2070, but slightly behind a 2070 SUPER or 1080 Ti. Like the 1080 Ti, that card also lacks hardware raytracing support, but can be found new for €100+ less than a 2070 SUPER.