Fleets acting as time freeze damage gates in Legends is a bad design choice.
Having Blackhole drop because it takes too long to kill fleets is bad. It is fine in mythic where it happens early and you know you need to plan on it - but in Legends having a completely viable build crumble around 900 waves because you haven't gone full GC yet is lame. It isn't like you need just a little investment in damage either - I have every damage mastery excluding beserker and an okay chain lighting + smart missile + spotlight - it just isn't enough to slay a fleet in the 900s in any sort of reasonable time span.
It isn't even that I can't survive or eventually kill the fleets - I just take too long and lose my permanent blackholes which eventually allows too much other stuff to slip through - and I can't even solve this with extra defense stats/modules because it sacrifices even more damage. It doesn't need to exist as a feature either because anyone who could do whatever 'infinite' style hack they were worried about is obviously going to be promoted to Mythic which will force them into a build switch anyway.