I don't think it's totally out of this world to think that this could actually happen. Adam Silver is big on having an all star weekend in a city where things are new, updated, and there isn't much construction currently going on, if any at all.
By 2020, all of the construction going on with the Millennium Tower, the Hub, and the subway at North Station should be finished...although the subway construction will be cutting it close. Plus, ideally this will be a legit contending team (hopefully) by then too, which only helps with drawing people in.
I actually think it could be more realistic to have it a year later, in 2021. It gives the city an extra year to clean up and get all construction done and plus, it would officially be the 70 year anniversary of the first all star game, which also took place in Boston. I can picture Silver falling in love with that selling point.