Using yet there is a dated/literary alternative to what would normally be expressed as "he could still have dwelt for a long time in that soft, well-upholstered hell".
As @200_success comments, it's a bad example for English learners. That could still be contested, of course, but I think this chart shows which way the linguistic wind is blowing...
That's not to imply yet should be avoided in general. Where it means [from some time in the past] up until now, it's perfectly normal ("I haven't finished writing this answer yet"). It's just not such a good choice these days where it references [some time in the future] after now, such as "I could yet finish soon". 1
It's also perfectly standard to use yet as a conjunction similar to but, but even that usage has at least faint overtones of "dated/literary/formal" to some.
1 Noting archaic/literary usages such as "I met him many years ago, when he was yet a young man", that should perhaps be "narrative reference time", rather than "now".