Let’s stop reviewing for publishers that profit from research! Join the strike!
https://josephpcohen.com/w/reviewerstrike/
And yes, there will probably always be people willing to do reviews for free. But if the quality/delay/expertise of reviews from commercial publishers starts to degrade, it encourages authors to look for alternatives.
Of course, for commercial publishers, there is an incentive to publish more -- so there is tension between taking more articles (more revenue in APCs, more volume, easier to attract subscriptions, more perceived impact for extensive properties like h-index) and being more selective (more prestige).
So worse reviews probably doesn't mean more papers accepted but a more random selection of accepted articles.
Assuming that articles have some intrinsic "quality" that most researchers would agree on, and that good reviews are better correlated to this quality than bad reviews, one would expect that getting worse reviews would make the journal selection process (even) more noisy. This can harm the prestige of commercial journals (if they start rejecting articles perceived as "good" and publish articles perceived as "bad").
Indeed, for-profit publishing only works because researchers are willing to do free reviewing work for such publishers.