Exactly the opposite -- in a lot of ways, mainstream languages represent a compromise that gives the worst of all worlds for the benefit of popularity. With AI, you can take advantage of unpopular esoteric languages that have measurable advantages over mainstream languages. Before, the choice for a mainstream language was too strong because of network effects. Now with AI those effects don't matter as much, so being mainstream isn't as much of an advantage.

The copious training data on mainstream languages doesn't really translate as a network effect, and therefore is kind of a red herring because the AI can use all that training on new languages as well. e.g. if you use a esoteric language that is vaguely object oriented, the AI will use its vast training on object oriented programs and apply it to your language instead of falling over because your language isn't Python and it doesn't have training data on it.

Moreover, the AI is trained on copious amounts of PL research, so it's not like the ideas in a new language are completely new to it.