That can be done easily enough with @media queries, I suppose. It's possible to send a table-less, fully responsive, modern HTML email that is invisible for less-modern clients. It doesn't require anything special and is possible today.
The problem is that people who send email need it to look a certain way, and leaving older clients behind, just like you'd have done in IRC/mIRC, is not an option. So having the option to use modern features doesn't change anything, at least for email.
> The problem is that people who send email need it to look a certain way
I think it's fair to say that actual people don't need any of this. It's businesses and advertisers that would be the overwhelming consumer of these abilities.
The problem is that people who send email need it to look a certain way, and leaving older clients behind, just like you'd have done in IRC/mIRC, is not an option. So having the option to use modern features doesn't change anything, at least for email.