It kind of sucks that having a defense objective get destroyed forces you to restart the mission, even while you have plenty of extra lives to spare.
You should try to use the extra lives to give extra chances to complete defense objectives. Like maybe it should have a big announcement about how the defense has been too damaged and consumed x number of lives to restore x% of health. Or maybe it could force respawn everyone using that many lives but restarting the section at a higher level. Either way, getting used to a level would help if didnt need to restart the whole thing just to figure out the ending.