No. You don't need "local currency". With widespread adoption you can just pay sellers directly with cryptocurrency.

You just moved the goalpost by miles.

The question is whether Bitcoin and Ethereum have provided value. "Have" means in the past up to and including the present.

You are implicitly agreeing with the statement that Bitcoin is only speculative, by admitting that it will only be useful without middlemen in a hypothetical future where it achieves wide adoption.

Nope. In reality, the tax office comes along, demands to see the seller’s records, and if they haven’t kept comprehensive records of all transactions and the fair market value of the coins at transaction time in local currency, then they get fined, or if they’ve been actively evading tax, jailed. Plus, even if they’ve kept records perfectly, they need to get local currency at tax time in order to pay their tax bill.

Bitcoin is useless for any legitimate business. The costs of using it vastly exceed any benefits.

A lot of dopes who think they’re smarter than the tax-man because they’ve been evading taxes by using bitcoin are going to have a ‘Martin Shkreli’ moment in the next few years.

It can be good for speculation/investment and that's legal and kind of legitimate.

And unless you're selling drugs or malware, you can't do that.