I don't want someone to have a poor team attitude and not care about what they're doing because they know they can't get fired for less than 3 infractions or whatever else the union negotiates.
I want underperforming mechanics out ASAP and the best ones rewarded.
That would be nice. I think it's more likely that it would be the mechanics that don't make waves and keep costs down who would be rewarded. Woe betide the mechanic who insists on delaying a flight and costing the airline large amounts of money to investigate a possible issue that might result in a 0.01% chance of a crash, rather than delaying it to a more convenient time.
My only experience is friends who have worked in unions and their stories make it sound like there's little to no chance for advancement besides waiting in line, so at best people skate by and do the bare minimum and at worst actively try to subvert management decisions.