Email is unreliable for communication, no because:

• can bounce around all sorts of broken internet issues for quite a while
• it's a formal internet standard that does not rely upon a single failure point like most alternatives to email (Facebook Messenger, Slack, Skype, Discord, etc...)
• yes there is no reliable "read" indicator, but it is the case for every app
• the error messages are more detailed than "failed to send"

Email the a one-line replies doesn't add value:

• it is the same for messaging apps :)
• email does not quash or control the types of communication you can send

Email always top-quote:

• you can reply the way you want. The message can be at the top of the previous message. You can also break up the original email and put the replies in-between.

De facto oligopoly of email providers:

• same arguments as for other messaging apps
• you can communicate between email providers, thing that you can not between messaging apps

Span and abuse:

• same as other applications
• but you don't have to do an outright block or mute notifications, you can put these emails into a buried folders or creating another rule: set up on auto-reply for it :)

Filters and organisation of email is often difficult. The tools provided are not always well suited for the task:

• first is only learning simple rules
• you are allow to filter and organize as you wish.
• you are able to switch of email providers if the tools does not fit for your usage.

Threading is unreliable:

• does not work at all on modern apps. Except Discord but it is confusing.

Email addresses are too tightly tied to your identity.

• Seriously ?
• You don't have to.

Searching is often inadequate

• Tru about 20 years ago. Now it is powerful!