When Walter, Peter and Olivia try to blow up the supply line from future Observers to "now," they aim to turn the wormhole into a black hole to wreak havoc on the other side of their supply line, expecting it to take the Observers years to recover. Since Observers in the future DO control time travel, nothing would prevent them from opening another supply line after those years of recovery into the very same moment in 2036, making the entire plan of shutting down the supply line ineffective.