OP
OK let me know if this is sanctimonious.
I don't think anyone has said that Westminster is anything other than an excellent school.
What I think , and some others seem to me to as well, is that Oxbridge entrance is not the only factor in deciding whether to send your son to a school.
You said "if all else is equal why would you not? "
IMO all else is rarely equal in these circumstances. You need to see whether the environment is one in which your son , if he were to be given a place , would thrive . There are other factors than the bald number of Oxbridge entrants.
Many other schools have great success in Oxbridge (and other top university) entry. A school where your child will work to their their best in
its culture , I would have thought, would have given them the best post school options . This is not wishy washy 'well as long as he is happy' speak.
If you have circa £165k to spend on his secondary education then you have other options which merit you both looking into. Westminster is fabulous , there are other fabulous schools.
If Oxbridge is your main criterion then I think people have just tried to point out it isn't quite as straightforward as you seem to say. IE there are 60 excellent and able girls entering at 6th form. So getting a place at Westminster is not just like lottery with a % of success , you have to be able in the first place . A more relevant comparison for you might be (excluding 6th form entry pupils) how many of the original cohort of boys got a place .
You say you are new to London , and good for you for asking , but if I were you I would ask around as you get to know people. You will find people who think Westminster is the best school in the world . You will find people who think it is not right for their child , you will
find people whose son was offered a place and didn't take it up in favour of another school. You will also find people who think e.g. Eton , St Paul , KCS , Winchester , etc etc are the best school in the world ) 5 years of his life is worth more thought than whether it's near Westminster Abbey and the raw Oxbridge data. (OK that last sentence might be sanctimonious 
If Statesmom Jnr is able enough , likes the school and gets a place then fab. Just have a look at some other options as well would be my advice .
The short answer to your question in your thread title is - It depends