That assumes they don't accelerate the accumulation of technical debt. For each item cleared how many new ones are added. LLMs accelerate your good engineers and your bad ones. So the slop likely increase faster than it can be cleared.
And it will affect good engineers and turn them into worse engineers too
AI benefits rely on these good engineers having 5, 10, 20 years of experience pre-AI designing (and fully, thoroughly understanding) these systems. What's going to happen to that engineering skill after 15 years of AI use?
It ought to only get better as it gets honed at an even faster pace than before, utilizing techniques and algorithms that would have been out of reach due to outside constraints.
