I don't think that declining mag sales led to less awareness of GTI International. Before forums, proper car builds would be debuted at Inters with most show goers been none the wiser about what had been created in someones garage over the past months. Now, people post every nut and bolt detail of their car build, even 500 pictures every time they wash their new car for Christ's sake, so the whole internet has seen these cars way before the day of the show. There's little point going anymore.
Those of us that looked forward to the proper old shows have grown out of that sort of thing, and the following generations don't feel the need to look at anything beyond their phone screens.