Many were skeptical when the NHL announced their expansion to the city of Las Vegas in 2016. The Sin City had no previous track record of supporting major professional sports, and commissioners of the major North American pro leagues were apprehensive because of the gambling scene, which many believed could tarnish the purity of games. With the success of the Vegas Golden Knights and the NHL in the city, the soon arrival of the Las Vegas Raiders of the NFL, and massive population growth in the metro area, it only makes sense that the NBA and MLB will soon place franchises in the city.
There are many reasons to believe that Vegas is already on the NBA's radar for future expansion or relocation of an existing franchise. The NBA already hosts their Summer League in Las Vegas during each offseason for recent draftees and league hopefuls. In 2018, the reigning champion Golden State Warriors played the Los Angeles Lakers in a preseason game at T-Mobile Arena (the home of the Golden Knights), which drew high local and national attention. With the arena in place for a team to play right away, a potential NBA ownership group would have a much easier time pitching to NBA executives why the city deserves a franchise.
For the MLB, it is not as clear whether or not Vegas is a serious target for expansion or relocation. The MLB season is 162 games, meaning 81 home games. With MLB attendance declining overall the past few years, the league may be apprehensive at expanding to unproven markets. Even so, there are some promising signs. The 2018 Baseball Winter Meetings were held in Las Vegas, giving the market exposure for top baseball executives. The Las Vegas Aviators, a Triple-A minor league affiliate of the Oakland Athletics, have the top attendance amongst Triple-A teams so far in 2019, even topping the MLB's Miami Marlins. It is clear that there is local interest in pro baseball, which could make the market much more attractive to the MLB.
Relatively, Las Vegas Metro is a small market at 2.23 million people in 2018. On the flipside, this is up from 1.95 million in 2010, and 1.56 million in 2000. With rapid population growth, there is greater feasibility to have all of the Big 4 pro sports leagues in Vegas. One thing that makes the city/metro distinct from other small markets is the fact that it is a major tourist destination. In theory, home games for either the NBA or MLB in Vegas will be filled by road team fans who are there to not only see their team play but also make a Vegas trip out of it, go to casinos, see shows, party it up, etc.
Another possible concern is that Vegas is an area full of transplants who have hometown teams that they are incredibly loyal to regardless of residence. Many have hypothesized that this would hurt attendance for a Vegas team regardless of sport. With the success of the Golden Knights early on, they have been able to build a large fanbase, including many transplants who have adopted the Golden Knights as their "2nd" team or "1B" team. It is not out of the question to say that this could very well be the case if the MLB and NBA come to the city.
With the more liberal attitudes towards sports gambling amongst the major pro sports leagues, and for the aforementioned reasons, the Las Vegas market is now much more attractive for these leagues, and the NBA and MLB could very well be in the Sin City by 2030.