As far as I’m concerned, higher compensation is a long term win. If new hires are offered less, they will know they are being taken advantage of.
For instance, they could tie pay / layoffs to seniority (all your coworkers are unfireable incompetent union “good old boys” that don’t bother pretending to work any more, creating a hostile workplace and tanking company revenue), or the contract could make it unprofitable or infeasible to continue ramping up production in the union controlled facilities.
Since Boeing managed to remove the clause saying they’ll keep making planes in Washington, I’m guessing the latter happened with this deal.
If you get paid well, have good benefits, and can't get fired arbitrarily (union still allows for firing based on performance), it turns out your workers will be happier and more productive.
Also, if labor costs (paying a decent wage) tank a massive jet company, they already had many other problems under the hood. Your argument is also proven to be false, because Airbus (heavily unionized) has been eating Boeing's lunch for years since they didn't greedily spin-off everything nor slacked on quality control for the shareholders for short-term gain.