Sad but he has so many valid points
Three years of treatment for a brain tumour, the global COVID-19 situation and the inability of Canada to send a full show jumping team to Tokyo factored into his decision.
Solid reasons. Other than covid, those reasons would not apply to most Olympic athletes. But each person has to make the best decision for themselves. 
I would imagine the other riders on the shortlist are thrilled with his decision. I’m not familiar with the selection process in Canada, but I would think it would have been hard for them to select anyone else ahead of him.
Hope his continued brain cancer treatments are successful and he attains good health.

I don’t think any rider is thrilled that he withdrew himself. They are a solid team and the reasons he gave (apart from his health) kind of put them in a weird position… and when he said he would not feel right getting a medal in the situation, he is right!
I doubt any of the other riders have a chance anyway… so why bother?
That’s also A LOT of travel for someone with a neurological condition.
Aside from the riding aspect, there’s the global pandemic and his immunocompromised system, the factor of throwing blood clots from long travel, and a number of other reasons. Not to mention… you’re thousands of miles away from your trusted oncology team… one medical setback and you’re in an unfamiliar hospital with potentially not insured healthcare options for very serious procedures. It is the prudent decision in every way.
Since the Canadian team did not qualify fir the Olympics I doubt any riders in Cañada feel anything but sadness at Eric’s situation. I believe they can send one individual rider and most people hoped Eric would be chosen. As he explained, however he sees himself as a team ride so with no team, he didn’t care about an individual medal. Best of luck to eric as he continues his battle.