Government always try to defile things that may potentially work against them or even threat them, so there's nothing special about government calling btc a Ponzi scheme. It's no Ponzi, but it's manipulated ,well, like any other asset in any market, the difference is that it's less mature so more prone to be manipulated. With all that said, btc is handling a torch of cryptocurrencies and help in crypto and blockchain globalization. There will always be hundreds of thoughts, but that's okay until btc make new heights cycle after cycle and complete its primary mission to some extent.