In Sweden (the largest industrial hub in Scandinavia), unions don't collectively negotiate wages with employers - they collectively set wages across an entire industry with the government and business management (think Syndicalism, which is the primary industrial philosophy in Europe compared to the US).
Also, blue collar membership has declined after the 2008 Financial Crisis dealt a killing blow to Volvo and Saab. Blue Collar union membership in Sweden peaked at 85% in the 1990s but has fallen to 59% in 2019.
Some American unions (primarily UAW and Teamsters) would have to fundamentally reform in order to act like European unions, and they would assuredly fight tooth and nail against such a change. Also, the kind of sector level salary negotiations common in Europe might fall foul of American collusion laws, and might anyhow be unpopular with local chapters (hypothetical example: if UAW and all major automotive manufacturers decided to set wages at South Carolina levels nationally, that would be a pay cut in the Midwest).
The characterization that they would eschew reform even when it would boost their popularity is not reflected in current events (certainly the managerial friendly unions of the past would avoid reforms)
At the end of the day, unions in Europe are much more pro-business management than any union in the US can ever dare.
For example, Swedish unions didn't go on strike when Geely fired thousands of Swedish autoworkers when acquiring Volvo, and German (yes not Nordic) unions haven't fought for salary increases leading to German salaries remaining stagnant since the 2000s, while helping German corporations like Volkswagen, Daimler, etc remain competitive globally.
In the US, unions will go militant and remain independent of the government and management because they are grassroots driven. In Europe, unions are subordinated to the government and management, as Union leaders end up joining both.
But, from the buyers perspective, I don't want union hands touching my car. I also will NEVER buy a volvo because they're bad cars from every standpoint except safety. This is the brand that though a turbo, supercharged hybrid drive-train would be a good idea in an SUV. The SUV isn't even that fast despite it!
But you can't, because you're wrong and you know it.
US car makers were always bad when it came to quality, unions or not since Tesla is no difference. Build quality doesn't come from "working ad hard as possible", in general quality is indorectly proportional to the amount of hustling people do, voluntary or othetwise. Build quality in anything assembeld comes from proper engineering, proper assembly procedures, properly trained staff, solid quality culture (guess where the Japanese traditionally excel) and a quality focused organisation. None of those have anything to do with unions, half-assery or anything else you brought up.
It's objectively better to work in a unionized environment. Let's you be "lazier" and get away with half-asserey - great from the worker perspective but from the perspective of a prospective buyer, ideally your product is produced by people who work as hard as possible. The Japanese have a work-hard without half-assing attitude. This is why Toyota/Lexus have consistency the best build quality, stitching, etc in their cars. Their employees are mostly not unionized and not even paid particularly better than the American ones - but the Japanese work ethic is preserved.
This is also why "made in japan" toyota/lexus is prefereed to made in america/canada toyota/lexus.