Hawkeye's not the most popular member of the Avengers, but he's got charm. And a solo movie would really showcase the strengths of the character in a way that the ensemble ones haven't.
Take in all that awesomeness.
The solo Hawkeye comics are a lot of fun! They also really stand apart from other comics in terms of tone. While most comic books still focus on large scale or epic heroism, Hawkeye's strength is in the quieter, smaller moments. Rescuing a beat-up dog and taking him to the vet. Covering his neighbors' rent so they won't be forced to move. They're simple acts of kindness, but they give us heroism on a more human level. I think the MCU could really benefit from telling a story like that.
Plus it would make Jeremy Renner happy.
Where is the downside?