Speaking precisely, a small minority is plenty. Successful businesses can run off a few hundred repeat customers and a bit of passion. The root cause is more that it is irrational to behave that way so there may be literally no or single digit people who will behave that way.

Putting some estimates to the situation, the calculation is something like (chance of a serious dispute arising = 0.0001) x (chance of a payout from the courts = 0.33) x (payout = $100,000) -> I'd be willing to pay $3.30 extra to avoid fixed arbitration clauses in a deal where there was serious money at stake.

That doesn't exactly capture all the game theory of the situation, but it suggests the risks here aren't worth shopping around for. It is easier and more effective to engage in economic voting with feet and punishing businesses that way for failure.