See for example this popular blog post: https://karpathy.github.io/2015/05/21/rnn-effectiveness/
That was in 2015, with RNN LMs, which are all much much weaker in that blog post compared GPT1.
And already looking at those examples in 2015, you could maybe see the future potential. But no-one was thinking that scaling up would work as effective as it does.
2015 is also by far not the first time where we had such LMs. Mikolov has done RNN LMs since 2010, or Sutskever in 2011. You might find even earlier examples of NN LMs.
(Before that, state-of-the-art was mostly N-grams.)
And what people forget about Mikolov's word2vec (2013) was that it actually took a huge step backwards from the NNs like [1] that inspired it, removing all the hidden layers in order to be able to train fast on lots of data.
[1] Yoshua Bengio, Réjean Ducharme, Pascal Vincent, 2000, NIPS, A Neural Probabilistic Language Model
[2] Yoshua Bengio, Réjean Ducharme, Pascal Vincent, Christian Jauvin, 2003, JMLR, A Neural Probabilistic Language Model, https://www.jmlr.org/papers/volume3/bengio03a/bengio03a.pdf