In actuality they can NEVER declare that the GT has started. Once they do, they have played their last card. Remember that. THERE IS NOTHING LEFT.
This is my own opinion, but I think declaring that the GT has started is a great card for them to play. 1.) Whether it has started or not is entirely subjective. 2.) They can claim that only God's people realize it has started (just like only God's people recognized that Jesus had started ruling invisibly in 1914). 3.) The GT has no time table assigned to it, so it can continue for years, decades even, as Satan and this system increase their attacks on God's chosen people.
The aim is to instill a perpetual fear of the GT and motivate the sheeple to be even more subservient.
Imagine the perpetual fear many JWs will have once they've convinced themselves they're now living IN the GT. "How can you leave now? We've made it this far! We're in the GREAT TRIBULATION!!!"
I personally think that they will announce in the next 5-10 years that they've "discerned that we are now living in a state of constant attack by Satan and his earthly representatives. These constant attacks led by false religion's leaders and governments around the world were foretold as a 'Great Tribulation' and will only increase and as we approach the Battle of Armaggedon. We do not know how long this Great Tribulation will last, so how vital it is that we remain close to Jehovah and faithfully obey any direction we receive from his organization." Nothing else will really change, but the GB will use it as one more "turn of the screw" to keep the pressure on and excitement up for a while.