I ponder this all the time. Short answer no, but the soul of it will decline (already has). The more salient of the points you raise is that, as Speed says, 'interesting' derelict places in a classical sense dry up, but RE a question of will future generations find structures built in 90s, 2000s, onwards etc exotic in the same way that we find structures of 50s and 60s vintage exotic - I think it's probably short sighted to say so. A post-industrial state will continue to churn over itself structurally, so there will always be something yes, just of applicable intrigue to those who have lived through a markedly different era. For now, especially those involved in the hobby in 2000s as well if not the 2010s, it's certainly famine rather than feast yeah.
Then RE the question increasingly secure premises, I don't think that will play as much of a part. If everywhere of interest to the wide umbrella of urban exploring is surrounded by three layers of electric fence and a hundred IR trip sensors, for example, I mean sure, the practice would wither, but there is no future economic scenario where anything close to this becomes a reality, so it's not really a question worth pondering. A landowner/landowning entity secures somewhere with his/its own capital based on how much he/it cares, and there just isn't enough money spread out around Britain to actually stop future generations (for whom disobedience is becoming far more normalised than it was for generations that preceded us) from poking about a steady supply of old warehouses or hospitals etc which will continue to find themselves in ruin for decades to come - K9 unit and rolls of razor wire or not.