Traditionally this has been the case, and largely because POS required such a large upfront investment that merchants needed to be sure (or be 'sure' thanks to

Traditionally this has been the case, and largely because POS required such a large upfront investment that merchants needed to be sure (or be 'sure' thanks to a product already having been established in the market) of what the details of the implementation are. When changing POS is as easy as uninstalling one app and installing another... UI will be a much greater factor. I expect many players to attempt to tie-in users through one method or another as a crutch for lacking quality UX.