>If we take that the liquor store that tourists shop in is the same liquor store/cafe/plumber that the locals use,
You can't make that assumption. There are a minority of businesses that are open in the off season that are used by locals. The majority of businesses are targeted at tourists only and any use by locals is incidental and immaterial to their balance sheets. Many of the business owners actually have other locations that they'll run in the off season (e.g. florida for the winter, nantucket for the summer).
Depending on the location (i.e. less true on an island where you have to incur a $100 ferry ticket to leave) many locals spend huge amounts of time traveling in order to buy goods at reasonable prices. Ecommerce (which doesn't generally change prices by location) and big box stores (which have rules about prices that franchise owners must follow) tend to be the places locals shop for goods in where possible. The further you get from pure tourism (e.g restaurant = 0 steps, restaurant's landlord = 1 step, landlords plumber = 2 steps) the more likely a business is to be open year round.
>I can't imagine anyone ever thinking, "gee, good thing the cruise ships dock in Barcelona,
Barcelona (or Miami for that matter) is lucky that it's a big enough city with enough other industry that normal economic competition keeps prices reasonable. Pretty much any city tourism destination will have reasonable prices there's other industry going on even if tourism is the big one. The quaint little towns with nothing but tourism are what I'm talking about. Sure, the locals who make their money close to the tourists would be angry if you parked a cruise ship there but there's also be a huge number hoping that the hotels and rentals lower their prices to compete with the cruises causing the restaurants to lower their prices, etc, etc so that they can actually justify the cost of going out to eat once in awhile.
>supermercados would go back to being astronomical and the garbage collection services would be able to take full advantage of us again"
The supermarkets where my parents live (they still live in the tourist town I left) fought tooth and nail to keep the chain known for low prices from setting up shop just barely within a reasonable driving distance. Many, dare I say most locals love outside competition because they can get more for their dollar.
I could tell you similar stories about damn near every type of business up to and including a scrap metal recycler.