https://www.politifact.com/factchecks/2017/apr/16/kirsten-gi...
https://www.lsu.edu/hss/english/files/university_writing_fil... (Page 5)
If I understand these summaries correctly:
1. Countries in Latin America are developing and therefore using more oil so they have less to share with us.
2. We are consuming slightly more oil.
3. Oil obtained from hostile and unreliable regions should be replaced.
The effects would be that oil consumed from the pipeline would be cheaper, it would be more reliable, and while not being totally for domestic use would make a significant dent in our oil consumption in the right categories.
Edit:
Opposition to the Keystone XL likely would have been more effective if it focused on (potential) environmental impacts as well as the spill that occurred in 2017. I think that's what the Biden administration focused on when it cancelled the permits.