First off, I haven't seen a single episode of How I Met Your Mother. But I am inclined to defend the choices of the writers. More on that later.
There's a great interview with Carlton Cuse in EW about the art of the finale, and specifically Lost's finale. Cuse, as you would expect, defends his and Lindelof's final episode. In a very eloquent and common sense way. He's pretty clear-eyed on the whole experience, maybe more than Lindelof, who seems to be traumatized by it, at least in Twitter form. He makes some excellent points: why are finales such a big deal? They're not only a big deal but they can make or break retroactively how the show is considered and tarnish the critical reception for all time. Why? Why is the final episode of a show the most important, as if the entire run of the program—60 or 100 or 200 episodes—has to be condensed into one and judged on that. The previous is my rhetorical question, not his. He talks about about the standard is different with mystery shows, like Lost was, where people expect answers.
I agree with him, and I loved the finale. But I sympathize with people who felt that they didn't get answers—only because the show made the questions so interesting and arresting. For a show about characters, as Cuse states, it did a superlative job with the fantastical SF/fantasy elements. But he said the idea was to bookend the series with Jack's eye—beyond that, they took the seasons at a time. They didn't have a blueprint for everything in the final episode, but they didn't want to do an explanation show as the end, so they focused on the spiritual and redemptive arcs of the characters, because they were interested in doing that. (Not in the interview but speculation: Lindelof lost his father during the show's run, and he kind of went from atheist to agnostic to Christian or Christian feelings, iirc from an old interview he gave.) They did do an explanation show of sorts with the third to last episode—about Jacob and the Man in Black's childhoods—but people hated that, so that confirmed their thinking that explaining stuff at the end wasn't the way to go. But, again, I understand G.R.R Martin's complaint.
Now we get to HIMYM. If the show was ambitious enough—romantic comedy crossed with temporal shenanigans—then I'm interested in why the the final scenes that covered years and the most important events took place in minutes. I don't think that was a careless or expedient decision. The writers seem to have known their ending from the start, so they planned this. They knew it would be polarizing but did it anyway. Why? Maybe they went too subtle, and a critical reevaulatiaon from the future will explain it. Alan Sepinwall believes that they locked themselves into a inescapable room when they shot the kids (make your own joke) in the early seasons, and then saw how much chemistry Ted and Robin exuded together. But even if they did—lock themselves in—that doesn't mean a good ending or art can't happen. John Irving writes a book always coming up with the last sentence first, and working backwards. It's a very interesting way of doing it.
I loved the Lost finale; I loved the True Detective finale; I loved the Breaking Bad finale; I did not like the Seinfeld finale. Not because it didn't tie up the character's fates—having them sent to prison was pretty clear and clever. I didn't like it because it was a glorified clip show. David might have been doing a misanthropic postmodern deconstruction of the importance of series finales—and if he was, I agree with the sentiment—but it just wasn't entertaining and fun. So I didn't like it, but that doesn't mean I now can't watch Seinfeld and that it's corrupted forever. It's just one episode of a TV show; it doesn't erase all the high quality that came before it.