Publisher consolidation is increasing.
From @davidacrotty:
The market share of the 10 largest academic journal publishers rose from 47% in 2000 to 74% in 2024 -- roughly half to three-quarters. In the same period, market share for the 5 largest publishers rose from 39% to 61%.
https://scholarlykitchen.sspnet.org/2025/08/20/2025-update-quantifying-consolidation-in-the-scholarly-journals-market/
These numbers are from figure 1 and should be read in light of figure 2.