The problem with CELTA/DELTA is that it is a "one-size fits all" approach,

Yes, that size being English language teaching, so what's your point, exactly?!

and it's true that if one holds a CELTA certification it is by no means a sign of professionalism.

No, but then what is?! Holding a CELTA/DELTA shows you have trained in ESL teaching and your conduct shows your professionalism. You can hold every degree and doctorate in your field and still turn up to work in your underpants, with a traffic cone on your head and tell your colleagues you think they're idiots. Qualifications and professionalism are not synonymous... And I don't believe anyone suggested they were!