It is unlikely that BTC will become cash, because BTC is still inadequate to put them into circulation in the consumer market. Too many people do not know how to use BTC technology
yes that's right, the lightning network is still not completely perfect. maybe 20 years from now it can happen. maybe 
if bitcoin becomes a currency it is quite difficult, because until now there are still many countries that disagree with the existence of bitcoin. should the government really believe with BTC they would agree with the existence of BTC. but it is all difficult to predict, because the government lacks trust in BTC.