I think any given model is going to decay over time. The data used in them becomes out-dated and the models cost money to run and various cost-saving short-cuts are thus made to reduce accuracy. Also, having your old model seem clunky can make your new model seem great.

Obviously, there are real ways new model get better too. But if we have diminishing returns, as many speculate, it will take a while for it to be entirely obvious.