If the OP is modeling the idea on the European football system, then there's no relocation. (Were you reading "relegation" as "relocation"?) Unless you're pointing to the fact that D(soon-to-be "G")-League arenas are too old and tiny to make money in the NBA, and thus suggesting that the NBA team should get the NBA arena. Arenas are indeed the main problem. It's a shame, though, because relegation battles are great for
(a) combating tanking (it would therefore remove the need for a lottery)
(b) making the end of the season as exciting for shitty teams as it is for good teams.
And hey, it works in the EPL, which makes a shit ton more money that the "Championship".
And this would be a way to get megarich assholes to buy D/G-League teams with the intention of losing a ton of money up front with the plan of getting promotion and making the money back.