I've been reading the largely negative reviews of the How I Met Your Mother Finale, and what strikes me about them is their use of the equation as the primary metaphor for explaining what went wrong: that somehow, for example, the quick ending of Robin and Barney's relationship negated the (admittedly) overdrawn buildup to their wedding.
But the deployment of that metaphor is perhaps precisely what the finale - and the show - works against. Whether it be the grand statement repeated throughout the seasons - that every relationship matters, that you can't just skip to the one that works - to the more precise principle behind, say, Barney's actions throughout the finale - in which his begging his friends to accept him as he is and allowing him to change in his own time in that poignant moment with the baby only works because we can't ignore how hard he tried for Robin - the show has worked hard to dissuade us of Barney's perspective: that relationships are meaningless until you've found the one you can't avoid, where there's no “progression” of character, only other situations to react to that ultimately happen in the sitcom-y out of time state. (In that sense, the finale addresses many of the complaints about how Barney became the main character rather than Ted: the show became, for at least one season, his sitcom, and it gave us a sitcom-ish ending for him).
To think that Ted asking Robin out after a life with the mother negates the really strong moments in the episode with the mother is to oversimplify: it's to fail to see that relationships can't negate, they only build. That love is messy, and complicated, and is precisely not like a sitcom insofar as the end of a relationship doesn't mark the eternal end to that relationship: people's needs and desires change, old lovers become friends or lovers again. That's true of the generation growing up in such an interconnected world. Such is true of Ted's favorite book, which he reads both on the train platform and to the mother in her hospital bed: Love in the Time of Cholera. The book describes a love which occurs before and after a perfectly good, life-long relationship, challenging the reader to surrender the sense that love is valuable because it only happens once: that it can be beautiful multiple times in one's life.
The railing against what seems like a typical sitcom ending, in which people end up with who they are supposed to, can only be reached by actually skipping over everything that happened in the show: in essence, by presupposing that it ends in a sitcom-ish way. Ted's kids don't need to know more about the mother; she was their mother. But we can see, in that scene on the train platform, the potential and the completeness of that relationship. I agree that the pacing may have been poor, but what would the alternative have been? To draw the relationship between Ted and the mother out, and then to kill her off? Wouldn't that be more cruel?
Instead, the more I think about it, the compression really works: it not only reflects the speed of later parts of life, but also gives Ted two endings that one is forced to encounter together (a play, of course, on the episode's title, which exempts the “nothing” from “Lasts forever” because what follows isn't nothing; we have to keep living without thinking of that living as negating what we did).
The series as a whole went too long; it absolutely repeated its storylines because it didn't have material past a certain point. But I do think the finale, with all its flaws, will eventually be seen as one of the best sitcom finales ever because it gave its audience more than most: with an unreliable narrator, a confrontation with time that was not merely episodic, and because it used (some) of its actors to the fullest of their abilities.