Laura's infirmity cripples Tom, stuck with the double responsibility of being the family breadwinner and 'supplying' (from the warehouse!) a suitable mate for his handicapped sister. Plus he has to put up with Amanda's eternal prattling about the glories of her youth and fuss over Laura's uncertain future. He is the 'chief cook and bottle washer,' a one-man-band, and finally cracks under pressure. Who wouldn't?
Tom confides to Jim early on that he has used money destined for the electric bill to pay Marine Merchant dues. Be it theft or treason, this clearly shows Tom's need and priority 'to get out' of a no-win situation and to find a life of his own.
Tom indeed follows in his father's footsteps by breaking away. Does he send money back home to still support his mother and sister? He does not say, but this would seem the most "suitable way" to meet his family's needs as well as his own.
Couldn't he have found another job or, as Jim, bettered himself by taking night courses? More than money or education, Tom needs vital space; for at home Amanda, though endearing, eats up all his air. As he takes the fire escape for the last time, his flight is physical but his "escape" is first and foremost a psychological one.
From Laura's standpoint, when she dances with Jim, though haltingly, something else other than the unicorn breaks - her inhibition and lack of self-esteem. She finds resources within herself 'to move on,' even without Jim.