I don't think it would be considered "stagnation" from the perspective of the old folks. They might see it as society retaining its sanity. From that point of view, this is not a cost, but a benefit.

In order to count it as a cost, you'd first have to assume that the values that future folks would hold, given no life extension, would be better than the values people would hold given life extension. What are the reasons to think this?