These DSLs are not there to replace templates. They are to define email lifecycle: those are different things. Email lifecycle dictates "who" gets an email (ie anyone with a Pro plan) and "when" they get an email (after they upgrade to the Pro plan). Template defines "what" they get (the email content etc). Those are different things.

You can do the "what" part (template) via WYSIWYG or django template or ReactEmail,... Sendlang is the equivalent of your django code but for the lifecycle part.