Many people do see that shortcut. But I don't think a writer has to write any female character any particular way in light of that presumed shortcut. You can't write to what's in the viewer's mind. There was no way that I can see that Adler could have been written that would have satisfied everyone. (I keep wanting to refer to the writer as Moffat, but IIRC he didn't write that episode. He and Gatiss both probably have plenty of input into the characters, though.)
As for women needing rescuing...it's not every show's every episode's responsibility to show a woman come out on top. But balance is nice.
Well, sure, I agree. But... balance where? Across all shows a writer/creator is responsible for? If we're judging the women in Sherlock along with the women in Doctor Who, as many people out there seem to be doing, as Moffat's body of work, well, River Song kicks ass, and the female companions have all been strong, smart women, with their own individual personality quirks. Bad things happen to them, but hell, bad things happen to pretty much everyone on that show. Or are we just looking at Sherlock for balance? Moffat/Gatiss have introduced more women than there were in ACD's canon, and none of them are dismissable. Their Mrs. Hudson is no pushover. The Lady in Pink was smart and level-headed enough to provide the means to capture her murderer even as she was being killed. Soo Lin was strong enough to get herself away from the Black Lotus and start a normal life. Sally Donovan is no fragile flower, John's ex-GF Sarah proved herself both brave and smart, and Molly Hooper is an excellent example of a woman who might not be strong emotionally but is a competent professional and a brave and loyal friend. I think they've done really well by the women. Is all that worth nothing because they made Irene a prostitute? Or because once in her life she had to be rescued?
They could have made Irene a high-powered international lawyer, or a trusted assistant or even a wife to someone in power, or lots of other things. All the plot needed was that she should "have something" on a member of the royal family. An affair would have done the job; she didn't have to be a sex worker. Once it was decided that she would be a sex worker, making her a domme put the aforementioned shortcut to good use -- she would not have looked so powerful otherwise. And she needed to look very powerful and very sure of herself if she was going to be able to hold her own against Sherlock.
As for why they made her a sex worker, I think they did that in order to explore Sherlock's sexuality, which they couldn't really do otherwise, because the character will not be having any sexual relationships. And making Irene gay leveled their playing field and made them, again, very much like each other - she was not attracted to him sexually any more than he was to her (arguably, depending on whether you see him as ace or celibate) - the attraction was something else; something both of them found more powerful and more irresistible than "mere" sex. Both characters disdained sexual attraction, he because it's a distraction to his work, and she because it IS work.
I thought the whole thing was pretty brilliant.