Looking the UK CAA numbers, there has be a clear change in number of passengers flown and which airport on the mainland they fly to/from since Penzance went off line. The total number of passengers by air to the Isles of Scilly in 2007 was 178,931 and last year (2013) it had decreased with 50% to 89,170 passengers. At Land's End/St Just the number of passengers has increase from 29,005 passengers in 2007 to 42,626 passengers last year (2013). Land's End's market share was 48% of all air passengers to Isles of Scilly in 2013 compared with only having 16% of the passengers in 2007. Even with two airfields merging to one, St Marys has seen a drop of 45,191 yearly passengers from 2007 to 2013. In 2007 65% of the passengers flew to/from the Isle of Scilly from Penzance (115,998 passengres) and I found it as a surprise that the majority of the passengers at St Marys flew with helicopter (53%) at that time.
The drop in passengers don't seem to stop at the 2013 levels. The first quarter of 2014 show a further 20% drop to/from St Marys.