I tend to agree, on the whole. LLMs force more attention toward the purpose of an app and how best to satisfy that purpose for a given audience. (Undisciplined SWEs can forget about end users and get lost in the details of some particular piece of tech.)

However, you could expect LLMs to train on existing successful apps and hardened design patterns that compose well into good UIs. In that case, you might expect the last mile polish to be the differentiating factor. However, the more marginal this last mile becomes, the more other social, political, business, etc. forces will determine which app rises and which falls. But this has always been the case, certainly in software.

To train for business and user outcomes LLMs need what they currently don't have: open insights into the real world app and business metrics.

If you meant training on UI and features of established apps then it gets even more complex, as success of B2C apps is obscured by their pricing strategies and marketing, and business models for successful B2B apps like Jira/Confluence/Office often revolve around sales, partnerships and other non-product things like that.

Any business is a set of design decision someone made, with their app just being a small subset of these.