I used to think the models got worse over time as well but then I checked my chat history and what I noticed isn't that ChatGPT gets worse, it's that my standards and expectations increase over time.

When a new model comes out I test the waters a bit with some more ambitious queries and get impressed when it can handle them reasonably well. Over time I take it for granted and then just expect it to be able to handle ever more complex queries and get dissappointed when I hit a new limit.