>= 50 % market share -> Sorry, you can't get bigger than that.
Actually I'd go further than just limiting consolidation, that is, forcing splitting (or converting to a public / regulated, profit-limited monopoly). That is, ensuring a fair deal to the consumer should trump shareholder interest when these are mutually exclusive. Split / conversion to utility would be a risk an investor bears, just as when something does not work and goes bankrupt, only here something was working too well for the company / shareholder and but not well for the consumer.
[Thanks for the new icons, BTW :) ... could use hammer and sickle here, but close enough I guess ]