Macron is not great, but if you dislike his reforms you'll have a fair chance at fighting them through the legislative process and the courts. A Le Pen presidency means the fight will have to be taken to the streets.
Hardcore Melenchonites need to take a long hard look at the impact of their choices and make peace with the fact that they won't be voting in a president but getting to chose their next adversary. That's some serious advantage for a political movement that didn't qualify, all things considered.
The lack of strategic thinking from some corners of the left is staggering.
In addition, even imagining the potential 'worst case scenario' outcomes such as 'Macron wants everyone to be an Uber driver' - You need to think about which legislative decisions can be most easily rolled back and repealed by the next President. As evident from the UK and USA experience so far, rolling back hypothetically terrible employment rules is going to be easier than getting back EU membership, catching up on environment protection and climate change.