I see in your case text was better performing than html, but I would not bet it's the same for everyone. It's just actually depends, and anyone with a simple A/B test will determine if text is better than email.

However the 5 reasons supporting text email are pretty wrong:

1) Poor rendering. Just use a good email editor, such beefree.io or others. Not a issue at all.

2) Mobile clients. Same as above

3) Email CSS. Same as above

4) Design effort. Yes, if you want a good result, you need a little effort. But if you rely just on text, you should put the same effort in choosing the best words. Consider than with a single Image you can communicate much more (and deeply) than with 20 lines of text.

5) Approval. Any communication department would require to approve email content, not only design.

Regarding the other reasons mentioned:

a) spam filters. Text doesn't change the inbox placement rate, the impact if any is below 0,001%, so totally irrelevant.

b) promotions tab. It actually depends on other variables, it's not a matter of text vs emails. If a transactional messages arrives in Promotions instead of Updates, you should fix the content (use a different IP/domain for transactional vs bulk), not the email format.

c) true, a template looks like something "bulk" or automated, while a simple text email may look like a personal direct email. Unless you want to cheat..