This show's lasting legacy may be that its the best lesson in not dragging out a show too long, and recognizing when the hang it up.
It was clear that they were stretching, and the "mother" twist became less effective the longer the show ran.
The showrunners likely felt compelled to keep it going because of money, and it's hard to blame them. But, the negative reaction to the final season torpedoed the shows dwindling reputation. While they may have gotten a payday attack the time, the showrunners have done very little since. This is during an era with the streaming wars when any showrunner with a decent reputation is commanding big dollars to crank out content, these guys seem unable to get anything out, despite the show being one of the most popular things on TV when it was on the air.
Although, I do think it's kind of funny that the failure of the planned spin off, How I Met Your Dad, likely saved Greta Gerwig from being tied down to as a sitcom actress, and instead let her become one's of the top rising directors in Hollywood.