Ive noticed after many years of playing with the current ship design system that the ai only seems to replace the ships once they lose them in battle, due to ship classes having a max tech contraint, this leads to sub par enemy fleets, for example its 2240, i have a fleet of constitutions and the klingons are still using d5s, I dont know if the dev team is working on it but I think if the issue was resorted the ai would be able to keep up with the player a bit better.