In theory, if everyone decided that starting January 1, 2021 all the previous academic effort devoted to Journal X could be instead turned to Open Science Journal Not-X, the impact on scientific endeavor would be essentially zero.
But getting everyone on board for that is a difficult task. Most faculty have no direct involvement in the allocation of subscription fees, so they don't really experience much cost (only that of emailing the occasional paywalled PDF to a less-well-placed colleague). It's easier to just keep on doing what we've been doing.