Santi, Rosicky, Mertesacker were all short extensions. Flamini was free and 29. Goalkeepers don't usually age as quickly as outfield players.
When has Arsenal paid money and given big contract to an older outfield player? The nearest is Arteta, who was 29 when we signed him. Modric will be 32 by the time. He'd probably be a great addition to next season and probably would be too slow after that if he ages as most footballer do.
It's sort of like United signing Schweinsteiger. A top playing winning the world cup and CL, shifted because he's losing his pace.