What you say makes a certain sense if a) you’re only considering large cities / suburb, and b) your have a pretty rosy unrealistic view of how much overt discrimination still goes on.
Most of the “public accomodation” demands the Feds have come up with are aimed at less diverse & less tolerant locations. IOW …
The homophobic baker located in/near the big city gay neighborhood will soon be destroyed by the marketplace just as you suggest. Or at least, another gay-friendly baker will appear to fill the vacuum and find enough custom to be successful perhaps alongside some number of other homophobic bakers.
Conversely, out in the rural counties that represent 80% of that state’s land area, 20% of its voters, but 70% of the legislative seats, substantially 100% of gay folks living there can’t buy a cake because substantially 100% of bakers are homophobic. And the few bakers who aren’t are so widely dispersed geographically that the would-be gay customers can’t find them. And they lack the market power in any one location to force a change in baker behavior via the marketplace. Nor enough market power to support a gay-friendly baker; especially not after the outraged straight boycott begins.
Ultimately it’s just another manifestation of the tyranny of a non-diverse and intolerant local majority.
That’s also why these kinds of rulings from the State or Feds rouse such local ire. The non-diverse population of rural county X sees no problem with letting their baker discriminate because there are no gays, or at least no gays who matter, anywhere within their emotional field of view. Even if they’re actually living next door.
But if “equal protection” is to mean anything practical, it means rural gays and straights are equally able to use local commerce to satisfy their needs & wants. Likewise and urban gays and straights.
And of course all the above is using “bakers” and “gays” simply as example placeholders for “any commercial activity” and “any minority however defined”.