There may be philosophical reasons to oppose a union, but contracting isn’t generally an issue. If you have more negotiating power than the union, you can just negotiate on top of the union contract, like sports and entertainment stars do.
Most software developers, even top performers, are not brand name celebrities. If the union forces everyone to follow a contract, they will need to comply. Realistically, tech unions will end up looking a lot more like journalist unions than football stars.
This is explicitly the goal of the new NYT union: they are looking to "begin negotiations for a contract with management."
Some union boss negotiating my employment contract is my worst nightmare. I have friends who work in union companies and they hate the union's policies but are terrified of going against the union bosses.
Basketball teams have a salary cap for their entire roster.