In reality, nobody is calling their new law firm a startup.

Nobody is referring to said law firms as startups.

Now - if it's a specialized kind of firm that sells services online, like a 'law marketplace' or something, maybe they will call it a startup.

Yes - the term startup is quite varied ...

However, the vast majority of business generally don't apply, so there is at least some kind of 'critical mass' to what we mean bu the word.

Generally 'a business' is not 'a startup'.