Vanguard Group Inc.’s biggest salvo yet in its campaign to cut fees for the investing masses presents industry rivals with a painful choice. Follow suit and lose potentially hundreds of millions in revenue — or hold the line and risk losing badly needed market share.

Even if they don’t cut fees, active managers need to determine whether prices are still fair and that their funds offer something that can’t be replaced with passive.

Vanguard Group Inc.’s biggest salvo yet in its campaign to cut fees for the investing masses presents industry rivals with a painful choice. Follow suit and lose potentially hundreds of millions in revenue — or hold the line and risk losing badly needed market share.

Vanguard made headlines by touting what it called “our largest-ever reduction in expense ratios.” The asset manager known for rock-bottom costs, said it was reducin