Maybe I'm missing the point here, but isn't it a big deal that Pelosi and members of Congress were planning this secret, overseas trip in the first place? Isn't this underscoring that they have absolutely no intention on ending this shutdown?
Other than the Federal employees themselves, the people in the general public most affected are the ones who rely on the government (read: people who vote for Democrats). At some point, they have to start blaming Pelosi, instead of Trump, for not getting their free handouts.
I think POTUS sending this letter is to highlight Pelosi leaving, more than it being an absolutely awesome retaliation.