RMT would move to a different item if shards were removed, but nothing else will ever be as effective for them. A level 50 crafter does not suddenly use more iron than a level 30 crafter, however they do use significantly more shards.

Mathematically, shard demand is a function of a static gathering rate with an increasing consumption rate. Shard gathering increases by about 20% with level (from elemental abilities), but shard consumption increases by over 500% - a bronze haubergeon takes 11 shards to make from scratch, but a cobalt haubergeon takes 65 (!). No other item increases in consumption so drastically.

The battle against RMT is one of making their business more costly to operate and less profitable. Shards are an easy target for them to exploit. If removed, the cost of their business would increase significantly, reducing the number of RMT that can continue to compete.