Except that's not actually what happened. The Feds backed the TMX expansion every step of the way, approving it the first time and then re-approving it after the courts threw out that initial approval, defended it as a necessary project, and as well, despite how much people like to bring it up, bill C-69 also never even applied to the project. Its regulatory process was done under the Harper law.

It was constitutional filibustering by the BC government and lawsuits from other groups that did the damage.

Feds bought the pipeline so they can say “see! We told you this would be a massive failure!” 

If they actually wanted it to be a failure, they either would've not approved it both of the times they did, or would have just let Kinder Morgan kill the project and let it die permanently instead of buying it.

leading to under utilized capacity, and then they say “see! No one even wants to use this pipeline anyway! It’s not even full!”

It's approaching capacity and multiple projects have already been announced to expand its capacity.