Because he's better than what we have. I'm all for modernising our football and have been on about it repeatedly. However it cannot be at the expense of actually doing your job. Smalling is comfortably better than Lindelof as a defender and it's hardly as though Ole is capable of turning us into some possession dominant City or Barcelona type team that dominates the game and usually 'shields' average defenders. Given our lack of tactical nous and dominance we need a proper defender. And unlike Lindelof, Smalling is capable of dominating, of heading and of not being a complete pussy.
I would say they have been “doing their job”. Only three teams with better defensive record. Of course it could be better, but it is clearly within “doing their job”. At least Smalling/Roma haven’t been doing a better job, despite the worse quality of the league.
Smalling would be good for rotation, but I don’t think having him as starter would improve things overall.
Many goals have come from individual mistakes, something Smalling , most likely, would not improve. It also seems as there have been less costly mistakes lately.
The only thing I could think of is that Smalling would be good at set pieces. On the other hand he makes many fouls per match and the number of setpieces would increase.
One important area for improvement is to further develop the ability to play through intense press. That would not be improved with Smalling, instead it would be worse.