IMO, there's zero doubt that Arlington and Watertown are suburbs of Boston by the most reasonable definitions of the word ("a. an outlying part of a city or town b. a smaller community adjacent to or within commuting distance of a city")

However, if you continue to object to calling them suburbs of Boston, then look at Carlisle, Dover, Lexington, Lincoln, Sherborn, Sudbury, Wellesley, Weston, Winchester, and on and on. None of those Boston suburbs are even hurting for money, let alone "insolvent".