I'd only really say monopolies are bad when free entry is restricted by threats of violence.

I'd also say that no matter how much market share bitcoin ever acquires, it won't be a monopoly, because there will always be the threat of entry by competing currencies.

What often goes unappreciated when discussing monopolies is how broad the scope is for consumers to substitute with other goods and services, or alter their lifestyles to reduce their demand. As long as people aren't forced to buy something, monopoly power is pretty limited.