the easy part is done but the hard part is so hard it takes years to progress
There is also no guarantee of continued progress to a breakthrough.
We have been through several "AI Winters" before where promising new technology was discovered and people in the field were convinced that the breakthrough was just around the corner and it never came.
LLMs aren't quite the same situation as they do have some undeniable utility to a wide variety of people even without AGI springing out of them, but the blind optimism that surely progress will continue at a rapid pace until the assumed breakthrough is realized feels pretty familiar to the hype cycle preceding past AI "Winters".