The most plausible reason they might need to dig them up is to remediate them. Landfills require maintenance in perpetuity, which costs a considerable amount of money. The biggest expense is maintaining the top cap—if it leaks, big problems can result.
After several centuries, it’s hard to imagine that most landfills will still be doing regular maintenance and fighting off entropy maintaining the cap. At some point, with the right technology, it becomes more sensible to reprocess the waste in a more permanent manner.
I think plasma gasification is likely the best idea, but it still needs work.
