To extend on what Jensson wrote:

A union has the power to organise one thing, to withdraw labour. In the industrial era, the threat of all the workers not showing up was a threat to end a business.

If AI does what is promised, to replace labour, then a threat to withdraw labour is only threatening the owners with a good time.

Unions doesn't give you power they just help you use what power you have. Unions don't help if you don't have any power, see Detroit factory workers, they were highly unionized but that didn't help them at all. And if you have power then you can start a union, so there isn't a reason to start a union early before you need it.