Good of you to suppose that engineers cognitive decline doesn't translate into long term impactful business challenges as well. I mean, once you truly don't know your product and its capabilities any longer, what's left for you to "sell"?
You are talking about productivity, I'm talking about knowledge. You may come-up with a product, then fire all engineers having built it. Then, what? It's not sustainable for a business to start from scratch every other year. Your LLM won't be a substitute for owning your product.
To quote myself:
> Companies don't own employees: workers can leave at any time.
> Thus protecting employees productivity in the long run doesn't necessarily help the company. (Unless you explicitly set up contracts that help there, or there are strong social norms in your place around this.)