Profitability doesn't matter if you're not paying costs but getting the income.

It does matter if you are: making pennies (Bitcoin on commodity machines is this way), are turning away users, or are using it instead of advertising.

There's also the bit about stability, you might have a business model that's profitable one month or worthless the next. Who wants that risk?

I believe the unprofitability is such that it can't even pay the expenses of serving the pages. Even if it could though, why would you opt for something that barely allows you to survive, rather than make a killing for all the value you are clearly providing?

They wouldn't be using Bitcoin, they'd use Monero.

It's free if it doesn't use your power though...

But it's not free when compared to advertising. Case in point, this article hypothesizes about botnets mining cryptocoins - but botnets are already frequently used to exploit advertising payment channels.