This is, of course, a matter of opinion. There is no way to know the answer objectively. My own view is that the future of unions is rather dim, particularly in the private sector. This opinion applies to some degree to all rich countries, but it is particularly true of the United States.
Unionization in the US private sector has dropped dramatically in the last few decades. There is no reason to believe that the trend will reverse. The main reason for this is globalization. Before globalization, American companies had much less competition from abroad. They could afford to have unions and to give unionized workers really good wage and benefit packages. Now, the competition from firms in other countries makes it much harder for American companies to afford the things that unions demand. Unless unions drastically drop the sorts of wage and benefit demands that they have been making, they will lose even more membership in the US as unionized companies fail and new companies do not unionize.
It is also worth noting that unions have lost some of their usefulness in other ways. In the past, unions were absolutely needed to force employers to provide better working conditions. Today, laws provide many protections for workers and so unionization is less necessary.
Unions may have a future as groups that ensure that employers follow correct procedures in doing things like disciplining workers. However, they do not have a bright future as groups that push for and win lucrative wage and benefits packages.