If he displaces Long & Obafemi then he become 3rd choice striker (perhaps 4th choice if you include Walcott), not beyond the realms of possibility. He possibly already has overtaken Obafemi in the pecking order or at the very least is competing with him.
We need players capable of performing in the Premier League, something he struggled to do. Scoring a hat trick vs Motherwell really doesn't help his case.
If the Telegraph are to believed that isnt true, as they stated Koeman was offered a huge wage to stay at Saints that would put him among the top earning managers.
https://www.telegraph.co.uk/football/2016/03/28/southampton-open-dialogue-to-extend-manager-ronald-koemans-contr/
Everton just offered more, along with a huge transfer kitty.
Sheffield United's Championship promotion squad was not made up of anything remotely close to a squad of £20m signings. Same for lots of other recently promoted teams.
Being significantly cheaper of course helps and brings more upside if promoted twice. But I'd guess it was at least in part because we have a bigger fan base and better infrastructure than all of them.