It will, ultimately, be a walkover for the supermarket. – if they want in they’ll get in. The notion of the creation of ‘local jobs’ is an easy sell. Most people don’t really understand the difference between a local job and local business and how one generates wealth locally and the other pays a wage with all the profits leaving town.
How it effects the rest of the town depends on what the town is like already. As a touristy town as you’re suggesting then some businesses – cafes, chintz, gifty stuff – will be unperturbed. Some on your better local suppliers – if you have good grocers, butchers, fishmongers etc, then unless they really do get tourist dollars then you’ll probably see them go to the wall. The smart ones will close their doors the night before the supermarket opens rather than slowly go to the wall.
Much depends on what the supermarket sells as well as groceries – will they do fuel? (your local shell/bp garage will actually be locally owned) pharmacy? dry cleaning? etc etc. If those businesses go – what fills the void? Will as many visitors come for tea and cake and giftys between empty facades?
The change for you as a resident is you’ll have town full of nice shops selling nice thingys that you won’t have a need to buy, and not selling the things you would.