These may all be fine arguments as to why a high-level(!) software developer might not want to join a union.

None of what you wrote applies to assembly line workers. And arguably, it doesn't apply to junior software devs either.

> I just need to convince the company to bump one total comp package, mine

> ...

> Also, I have an easy time convincing the company that I am leaving (or not joining), if they don't accede to my demands.

And when your aggressive negotiating is too much of a hassle for the company they just have to fire one "problematic" employee, you. And unless you have created a hellhole of a system architecture in your project that only you can maintain as insurance/backup to your aggressive negotiation, you will be surprised to see just how replaceable you are.

To be honest, I do most of the aggressive negotiations before I join the company.

So if that's too 'problematic' for them, they don't need to hire me.

If you want to negotiate just as aggressively while working for them, you have to have your outside options all lined up. Basically, yes, if you threaten to leave, you should not be surprised if they call your bluff and take wish you farewell.

Always be aware of your BATNA https://en.wikipedia.org/wiki/Best_alternative_to_a_negotiat... and make sure it's a good one.

Unions threaten that all employees will quit all.the.time. It’s called a strike. It’s their primary bargaining power. Yes, the workers will eventually come back to work, but only after they gain some concessions from management, and it can be a long time before they come back.

I know about strikes. They are different than everyone quitting.

> Yes, the workers will eventually come back to work, but only after they gain some concessions from management, and it can be a long time before they come back.

Workers don't get paid while on strike (they get some money back from the union that they saved there themselves). There's no guarantee that management will make concessions.

Yes, it can be a long time before workers come back.