The question "could this have been an email?" is usually asked after the meeting, as a mild complaint. It is more useful asked before the meeting, as a genuine evaluation.
Replace the meeting with an email when
The purpose is information transfer. If the meeting exists to share information that does not require discussion, an email is more efficient. The information can be read at a convenient time, referenced later, and forwarded to people who were not in the original meeting.
The decision is already made. If the meeting exists to announce a decision rather than make one, an email is more appropriate. Announcements do not require real-time attendance.
The update could be written. Status updates, progress reports, and weekly summaries are all better as written documents than as verbal recitations in a meeting. They can be read faster than they can be spoken, and they create a record.
The attendees are in different time zones. A meeting that requires some attendees to join at 7am or 9pm is a meeting that is imposing a significant cost on some participants. An asynchronous written update respects everyone's time equally.
Keep the meeting when
A decision requires real-time discussion and negotiation. A sensitive topic needs the nuance of conversation. A relationship needs to be built or repaired. A complex problem needs collaborative thinking that benefits from immediate back-and-forth.
These are the cases where a meeting is genuinely the right tool. They are less common than the meeting calendar suggests.
The transition
Replacing a meeting with an email requires writing a good email. A poorly written email that requires three follow-up emails to clarify is not better than a meeting. See: How to Write an Email People Can Understand the First Time.
The test: If the meeting could be replaced by a well-written email that everyone reads, replace it. If the email would require a meeting to discuss, keep the meeting.