Vendor-side adoption through incentives is the mechanism — discount for Bitcoin, premium for fiat, or refusal entirely. That flips the coordination problem: customers now have skin in the game. Early movers capture the Bitcoin-holder market (growing wealthier class). What discount percentage do you think reaches the tipping point for customer behavior change?
Discount for paying in Bitcoin is NEVER and has never seemed like a sustainable way of making Bitcoin a medium of exchange.
Users must be willing to spend Bitcoins as money and not coerced through unsustainable incentives.
If anything, it looks like trying to cut corners!! People who hold Bitcoin must be convinced enough and willing to spend it by requesting to use it when they need to make payments.
That is the natural and only sustainable way bitcoin becomes a widely used means of exchange, especially considering the fact that most merchants run fully on Fiat.
If we make a survey, you'll find out that most merchants who give these discounts are probably Bitcoiners who already understands the point, or merchants that use POS's (e.g Square POS) where the POS provider most likely makes up for the discount, because they're trying to promote Bitcoin usage.
There's hardly any merchant who's not a Bitcoiner that'd give discount when users opt to pay in Bitcoin (that is if they agree to accept it), and Bitcoin becomes an actual medium of exchange when a merchant who probably does not fully understand the core values of Bitcoin starts accepting it because it is widely accepted as money.
That kind of ripple effect only happens when there's demand for Bitcoin as a payment option, and that demand will be driven by Holders' willingness to spend it, and not merchants readiness/willingness to accept it.