The flaw in your thinking is that unions aren't just normal organizations. They have their own laws.
This is not a flaw of unionization or a valid argument against unions, but go ahead and argue against the regulations. I personally would support removing any laws that protect unions specifically, as many states have done with their right-to-work laws, but if and only if we also remove corporate personhood and other corporate protections. But that is beside the point, since supporting unionization has nothing to do with supporting current regulation.
You're very casually dismissing a hundred years of violence and intimidation.
If you were to add up the total number of deaths directly linked to unionization it would considerable lower than many other organizations and actions. The most obvious is the military, and few people would support a wholesale disbanding of the military. Second would be corporations, with disasters like that caused by Union Carbide out numbering the union actions in one incident. Nuclear power, the war over oil, tobacco, alcohol, suicides from the stress of higher education, and these are just a few off the top of my head. These are a mix off good and bad things. The point is that unionization is not amoral, any more than say something like religion. Sure the inquisition killed numerous people, but the quakers and the Mennonites have not hard any one (with a couple rare notable exceptions). The unions of today are not the thugs of the industrial revolution, and even then most union members where moral and law abiding citizens. And that's not even talking about the number of lives saved through improved working conditions that was brought about by unionization.
Seniority is not a valid basis...because they can't be let go
Your are mixing up loosely related issues. Seniority is a valid basis if you chose to only keep competent and productive employees. The fact that getting rid of incompetent employees might take a bit of a paper trail is no reason to argue that seniority is invalid. In most states you don't even need a paper trail. Sure the unions are there to protect employees from wrongful termination, but trust me, if the person deserves to be let go the unions will not protect them. The idea that unions protect incompetents is a myth, intentional or otherwise, that undermines the reality of unionization.
Keep believing the myths if you must, but I would suggest that you educate yourself, with first hand experience if possible, before dismissing the benefits of unionization.