When he bossed games under Van Gaal, people then also proclaimed that it was only due to tactics, which made defenders look better than they were. We mostly played possession-based football with high-line defence. Being aggressive and on the front foot - intercepting using his physique made him boss games. Using his pace for recovery runs collecting balls in behind made him boss games.
He posses more qualities than you portray here. Yeah, he is good in the box so he is being used like that, but he is not making the team play that way. If you watch games you will see he sets the line and is always quick to push the team up the field and out of the box. Playing beside the likes of Jones, Bailly, Rojo... Mourinho has opted to use Smalling's defensive versatility to make him the deepest one while his partners the most aggressive. That doesn't mean he only is good for clearing crosses. He has proven time and again he can play to a good level in a high line defence.
Btw, intercepting well and giving quick short passes to midfield on the break like Smalling and Jones did today is way more beneficial to our attack than playing hollywood-balls when in possession. That is just one way a well-resolved defensive situation can provide in an attacking sense without actually having defenders being midfielders.