But couldn't, and wouldn't you expect, both things to be true? Existing memories may be degrading as a result of ongoing neuron death and synapse pruning, but

But couldn't, and wouldn't you expect, both things to be true? Existing memories may be degrading as a result of ongoing neuron death and synapse pruning, but at the same time our "storage capacity" (neurons+synapses) is essentially fixed or decreasing, and new memories are going to increasingly be competing with existing ones. Presumably there is at least some connection between our neuron + synapse counts and species longevity. If we had evolved to live longer then one might naively expect us to have more neurons/synapses to support life-long learning and memories, although you could also argue that, for example, episodic memories too far back are not useful to remember, so maybe there is a cap to how much episodic memory capacity we need, and the complexity of the evolutionary niche we inhabit also limits the amount of declarative knowledge we need to store. It's been noted in birds that hippocampus size seems highly correlated with the species memory capacity - but specifically for memory of seasonal seed hiding locations, where the bird only needs memory capacity for the number of seeds (can be 10's of thousands) hidden in a single season - they presumably reuse this hippocampal memory capacity each season, and forget the last season. Have there been any studies relating brain size / memory capacity to the need of a species for lifelong memory?