Not true - at some point existing units use too much electricity and value of new BTC mined cannot cover the cost (past efficiency is irrelevant).
That won't be for a while even assuming no price increase. Even still as the oldest least efficient becomes obsolete it will be resold to those with cheaper power and when it finally goes offline there will still be newer more efficient gear coming online. net-net you may see the hashrate stall but a sustained down trend? Not likely.