After rewatching the series, I have a theory. Ted's and Tracy's relationship, where they were able to truly fall in love, get marry, have a family, etc; it only worked in large part was due to Robin not being a part of their lives. From 2013 to 2016, Robin was married to Barney and she was travelling place like Buenos Aires, Argentina for her job. Also in 2016, after not seeing them for a long time (gap of months between her telling the gang she and Barney was getting divorce) Robin didn't want to hang out with the gang anymore because she didn't feel comfortable about it. It was not until 2020 when Ted and Penny was talking about the GNB tower when they ran into Robin after a long time. She said she will move back to the city more permanently. This means from 2017-2020, she was not in town or even in the country, and no where near Ted and Tracy. And by then Ted and Tracy were about to get marry and do get marry. All this to say is, notwithstanding the similariities between Ted and Tracy and their romantic relationship, I think if Robin was still hanging out with them, things may turn out different. Robin may have cause Ted to keep falling for her. Tracy may get jealous and be like Victoria and gave Ted the ultamatum: it's her or Robin. Their relationship may have been shaky. What does everyone think?