Responsive Email Layouts - Why move away from Smarty?
You are right, we do need some templating system and we don't need to replace smarty. It's just that it's not the framework that seems to have a lot of traction these days and as generally, we are thinking of moving away from it, we might as well start with the mail feature

Moreover, I do recall performances issues being mentioned for big mailings (smarty was generating tons of temp files), so that might be another incentive.

This being said, I the focus should be on improving the layout of transactional emails and make it easier to brand them consistently for an organisation, so replacing smarty might not be super useful in that context.