Even with the potential of Wanyama, should he sign for Saints, there is plenty of room in the line-up for places. What is needed is more competition. Given Wanyama's seeming versatility, it would appear that any of the midfield positions could be covered if the need arose. Last season, the problem was of insufficient quality in the side when injuries and/or suspensions occurred and sufficient quality overall. In one full swoop, Wanyama adds to the competition and raises the bar for the others. It also means that Saints can look to competing in Europe in a couple of seasons, should things go to plan, with an already strengthened squad that will need less additions than if little or nothing was done now.
In short, Saints are just getting enough quality in to make the leap up the table. I'm sure there will be one or more additional signings to come, well before the season gets underway.