not until the movie at least
Even then, I think Thanos' overall popularity will finally blossom enough to sustain the book as a major key, thus helping it retain its value
Villains never hold their value well maybe a couple months after the fact then it's a downward trend.
NEVER, that is not true at all. Characters like Thanos and Darksied have a cult following and always will. Me personally I always root for the villains i.e. Darth Vader and have been a huge fan of Thanos for a long time and would pay a premium for his books over heroes from that time period anyday.