I think this could reasonably be addressed, and several startups have. The trouble is that the default email clients (gmail, outlook, etc.) don't really try to make it any better.
I've also generally had the opposite experience, a huge amount of business offices live and breath in email (mostly Outlook, but I'm sure it varies). Startups tend to run fast and lean, but as soon as you have some threshold of people, email is king.
We used outlook and slack. Business primarily operated via outlook as most communication was unsurprisingly external. Most but not all internal was slack.
I'm not hating on email, it has a lot of good properties and still serves a purpose. Every office appears to have some kind anti-slack vigilante. It's really not that bad.
I've also generally had the opposite experience, a huge amount of business offices live and breath in email (mostly Outlook, but I'm sure it varies). Startups tend to run fast and lean, but as soon as you have some threshold of people, email is king.