You might ask "why don't they just re-bid the contract?" Answer: The new 'progressive' city government is opposed to building a train for 'rich people' since there's already a non-express metro that goes through some of the most crime-ridden neighborhoods of the city. Of course no businesspeople use it and they all take Ubers (many of which are Teslas)
The last time I took the Blue Line to the airport it was indeed fast and convenient - but dealing with luggage was kind of annoying, and there was a guy smoking weed on the train. If I were traveling for business there's no way I would use the Blue Line unless it was peak rush hour, and indeed most business travelers don't. Getting weed smoke on your work clothes is a less than ideal way to start a trip, nor is getting sweaty in your work clothes from hauling luggage around the station.
Thus the case for an express train - it would capture those price insensitive business travelers. You could charge $20 a ticket and it would still be a significant savings over Uber. Heathrow Express in London is like this and it seems to work well.