Statistics are deceptive on this since nominal growth in output fails to register the poor quality of the goods being produced (to the point where even newly made goods needed to be repaired, and much of output was simply useless). The economy's failures were (within the constricted terms that anything was debated) widely recognised since at least 1977, and stagnation was first named, not under Gorbachov, but Andropov, and by economists like Fi'lev and Zaslavskaya recognised as a systemic failure.
By the early eighties, everyone knew that the Soviet Union was royally fucked. You can't get away with pretending that Gorbachov and Yeltsin somehow magicked away the fantastic success that the USSR was - they no doubt played their hands badly, but the hand they were dealt by those that preceded them was truly pathetic. In truth, the problems in the Soviet economy were already deeply embedded under Stalin's 'command' economy, whose pisspoor organisation was even less responsive to demand than the market.
There is a reason that the USSR lost the Cold War.