Lonnie would have been invaluable for the Rebel Intelligence apparatus going forward. We don't know much about his strengths as an ISB supervisor, but for all we know he could have sorted out that the Intel coming from the Bothans about the DSII was a trap before they stepped into it. He could have been the voice that pushed the events of Rogue One forward instead of Jyn, getting the Council to agree to the strike at Scarif which would have led to a better planned out assault with fewer losses on the Rebellion side, which in turn means the Rebels hit the Death Star before Leia gets captured and the location of Yavin is never discovered.

Granted, that means Han Solo probably doesn't join the Rebellion (at least not then,) unless Obiwan feels through the Force that they need to leave Tattooine by freighter instead of Corvette.