Email is very useful but does not make money. Salesforce, which is probably easier to execute as a product than Gmail, makes much more money with fewer users.
It's not surprising that they die off all the time as there is no way to monetize them.
History seems to prove your point. At Missive, we add features on top of emails (collaboration). Our goal is to create enough value to justify a per user monthly fee.
It's not surprising that they die off all the time as there is no way to monetize them.