Roberts was right to shoot down the Democrats' invitation. Congressional Democrats have had it in for conservative justices for years and even used a portion of their letter to Roberts to complain about Justice Clarence Thomas in addition to Alito. Democrats are seemingly never going to quit pursuing these justices over nothing, and Roberts is absolutely correct to not indulge them.
Roberts has again proven why he'll probably go down in history as one of the most problematic chief justices ever. Even if he doesn't deem it wise to meet with Democrats, Roberts could at least take steps to implement strict ethics rules for judges. This would reassure legislators and the public that there won't be any improprieties involving future cases. Instead, he continues to stymie every attempt to hold SCOTUS accountable.