The kids thing wasn't even the deal-breaker for Ted and Robin in my eyes. That's not the thing that made the ending feel so hollow.
What made it feel so hollow is that Ted and Robin, to me, didn't seem to really "get" one another. Not in the same way Tracy and Barney got them, respectively.
I know that's all extremely subjective, esp. considering how all-over-the-place the chracterizations have been ever since this show stopped being great. Ted in particular suffered. The lovable, eccentric, and slightly dorky romantic of the early seasons was more and more turning into a pretentious superdork. By the time The Mother was introduced, it seemed hard to imagine that anyone would actually dig Ted's many quirks ... but thanks to Cristin Milioti's fantastic portrayal, and some bits and pieces of decent writing, it felt to me that The Mother really did. Robin, on the other hand, felt like someone who was willing to accept Ted's quirks, because under all the dorkiness, he was still a decent and reliable guy. For Robin, it was worth putting up with Ted's quirks. The Mother actually thrived on them.
(Yes, this is all pretty Ted-centric - to me, Robin actually became *unpleasant* with the character assassination the writers dumped on her in later seasons. Seriously, what the hell was up with her treatment of Patrice?)
You know, I've actually had a Ted-Robin relationship. There was nothing wrong with it. We got along fine, we still do. My Robin and I get together every few months for coffee, beer, or shots, depending on our respective mindsets. We inevitably end up talking about the romantic misadventures we're going through now/have gone through since. Their parallel nature makes me wonder if we subconciously agreed to subject ourselves to the same mental injures at the same time.

And whenever we discuss the wounds we've suffered and/or inflicted on ourselves, we both start thinking about how much easier it'd be to just say "oh, screw it", hook up again, and give up on the whole idea of ever having the kinds of conversations Ted and Tracy had under that umbrella. My god did the conversation Ted and Robin re:"If we're both in our forties, and [...]" remind me of her/us. But, we've both so far managed to remember that, to paraphrase Barney, that isn't the dream.
Now this'll come off as a bit melodramatic, but regardless of how awful HIMYM has mostly been this last year, it's kind of helped me deal with some shit in my own life - or at least has given me some temporary relief from it. Last August, my then-girlfriend, after almost three years, revealed herself to be more Stella than Victoria, and left a gaping hole where my heart once was. Said hole is still there, and will be for some time. In trying to prevent myself from becoming a complete cynic, I actually *really appreciated* glimpses of the Ted-Tracy dynamic. They felt so real, and so relatable - even for a no-longer-that-young eccentric romantic/pretentious dork as firmly stuck in a my-insides-are-dead-and-I-will-never-love-again mindset as I was/am.
Yeah, I know nobody can/should demand that fiction tells the stories you want to see/hear/read, but given all that, there was no way for me to feel about the reveal but ... sad.