We all know that the development of a certain country comes from the taxes of its citizens
You also need to consider the business that are established within a certain country's soil are obliged to pay taxes to the government and isn't exclusive to the working individuals/citizens of the society. The taxes of the citizens contribute to a large portion of a country's budget but it isn't the only source wherein the government get its funds from.
and we all know people who engage bitcoin don't pay taxes to the government, what will happen to a country where 80% of its citizens engage in cryptocurrency?
Wrong. Places like the US tax their citizens who engage in crypto-related activities, so as some countries in the EU. If ever a country has 80% of its citizens engaging in crypto trades, I know for a fact that before it goes widespread, the government would already have imposed tax rules regarding that so no one would ever use crypto as a safe haven to launder money and a means to evade tax.
There is a big possibility that this things might happen In the future because bitcoin is growing faster and very known today.
I hope so, but on the way things are right now, it seems quite improbable for the said scenario to happen even in the coming 10 years to happen. People only use bitcoin to amass more fiat whether we like it or not. They just use bitcoin as a stepping stone for fiat riches and nothing else, so with it, they'd probably ditch bitcoin after they've milked it well enough.