I can spend USD almost anywhere I want at maybe a 1.5% transaction fee depending on merchant, with 1%-2% cash back (I calculated a 1.67% rebate for 2019). Technically this is only nominally government currency since it's just bank credit denominated in USD.
Bitcoin's blockchain does around ~100K BTC in volume a day at ~100 BTC in fees and ~1000 BTC mined (since May) which both are effectively costs going to the miners' electricity. 1.1% vs 1.5% with cash back isn't a huge argument for bitcoin. If transaction fees stay around .1% until the block reward hits .05% then the efficiency argument eventually goes away because miners will be competing on transaction efficiency and not block rewards any more.
I like the technology; it's cool and the fact that it works at all is amazing. If overall costs were closer to .1% and speculators were not the primary users it would be revolutionary.
It already works. https://why.cardano.org/en/introduction/proof-of-stake/
The speculative bubbles are a bigger problem. Bitcoin looks like at best a $5K-$10K thing in the long term (just look at the 2018 bubble). The argument was always "with limited supply bitcoin price will go to infinity!" but there isn't limited supply because bitcoin isn't the only cryptocurrency. The Bitcoin-cash split doubled the supply. Every new cryptocurrency product increases the available supply. Smart investors will diversify cryptocurrency holdings and bitcoin will eventually be priced accordingly; the cryptocurrencies that can grow appropriately with the economy will have the most stable prices. At least difficulty adjusts more frequently than the block reward decays so that it's very unlikely to have a giant crash where over-invested miners all go bankrupt on a downturn and no one bothers to mine the next block.