For most people email is a utility. The diversity of language in the middle chunk of the distribution isn't very high today and the biggest complaint most people have about email is how much time it takes. Most of our users aren't writing poetry, they are doing every day business transactions and we can help them be much more productive. Think how many times you've written "hope to hear from you soon", or some equivalent.

For the tails of the distribution Smart Compose is not helpful. We address this primarily with a triggering model, we aim to only show suggestions when we're quite confident that you're in the 'just getting things done' mode.

And if you're the type of person who always has a lot of personality in your emails, the feature probably isn't a great fit for you. Today it's opt-in, and there will always be a setting even if we turn it on by default.