Competition from non-junior devs, who can do all the same things the junior+GPT can do and also the tricky parts which "GPTs" can't yet do; and also have the benefit of domain-specific expertise about some area of business and/or better connections for investors, marketing, B2B connections.

This hypothetical scenario is literally like "pull up the ladder behind you", as all this experience and connections is something that a senior person has gotten while being handsomely paid for their time, but a future junior person may have to get on their own time and dime.

Ideas are a dime a dozen, execution is everything, and there's no reason to assume that random unemployed inexperienced people will be superior at execution.