I think YABU yes. At 45 there is a hugely increased risk to the baby of Down's Syndrome and it's not like you don't already have two children so why tempt fate unnecessarily? Of course if an unexpected PG happens then I would understand anyone who felt compelled to continue with it, Down's or no Down's, and absolutely no judgement from me. But why actively try for a third child at 45 when the odds are stacked against that child for a life without disability? It strikes me as playing a self-indulgent game of Russian Roulette.
If the child were to have DS or any other serious disability, by the time they are about 30 the care and responsibility for them may well fall to their siblings if you are not well enough or fit enough (or even alive) to care for them yourselves.
Your existing children will be only in their early 40's by the time you will be 80 and they will be at their busiest and most fraught juggling their own young families, mortgages and careers plus elderly parents. Why risk placing an added burden on them for something they didn't sign up for?
I am sure many people with disabled or vulnerable siblings who can never be independent would take on that responsibility in a heartbeat and without complaint because they love their sibling - but would you risk putting that burden on them, given a choice?
Also, as a rather depressing aside, (and sorry to be such a doom-monger but I have to mention this) I am 51 and in the last year I have seen:
four old friends/acquaintances in their early fifties die of cancer
one 50 yo female friend with kids still in school have a massive stroke
one 46 yo with youngish kids diagnosed with a brain tumour
three others from 44 to 54 diagnosed with cancer, one of whom will almost certainly not be around in a two years time and she has children of 11 and 13.
Of course no-one can foresee things like that but seeing just how many people don't get to make old bones, it's made me really reflect on the recent trend for planning/choosing to have a family later and later, just because you can.