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 race to the bottom on price, active managers, in particular, still need to ensure their prices are fair and that they offer something that can’t be replaced with passive.

Asset manager Vanguard said on Monday it was lowering the cost of investing across its fund lineup in its largest cut ever and estimated that this would translate into savings of more than $350 million for its investors this year.