I wouldn't say it is pointless to fight them. If you don't fight them they'll kill you. If you defend yourself, fighting them over and over, it is far from perfect, but you will live.
Also: I don't know how fast they respawn in the realm of chaos. Maybe if you kill a Great Unclean One it takes them years to resapwn. Might be worth it to snipe them out so you can more easily kill their army that is now without leadership.
Seeing as chaos will ultimatly overwhelm everything it is. Chaos as an enemy is an unstoppable inevitble force of nature. They're sooner going to stop the heath-death of the universe than stop chaos in 40K...
As for respawning the realm of chaos doesn't care about time, might be a millenium, it might be 1 second..
To be fair, this is the point of chaos. It is an ever present threat. Without it, the stormcast would win, the realms unified, and noblebright would be a thing.
With chaos, there is conflict everywhere
There's destruction & undeath to be a threat too. Plus, noone says all of order needs to be on friendly terms with eachother all the time. Have the dwarfs and elves fight a cataclysmic war for a while, plenty of story there....
And chaos could be fine as an ever present threat without posing the ever present threat of a freaking apocalypse. A mortal khornate army rampaging throughout the lands and murdering people is plenty of a threat without it also being a herald of the apocalypse. It's the fact that it's an everpresent, and ultimatly unstoppable, threat of apocalypse that just makes it all so pointless.
Same with Tyranids though - if an army is destroyed, more will be spawned. Besides, when the Daemons do return, the Tyranids would have moved on after devouring Shadowbrink, so there would be nothing except a barren rock for the Daemons to corrupt. Not much of a playground for them...
Tyranids, just like any mortal race, can ultimatly be defeated though. Their resources are finite. Defeat enough Tyranid armies and eventually they have no biomass left to spawn more. You won't be able to do that with chaos. Hell even if you'd permanently destroy demons, or even the actual chaos gods, you'd barely make a change given how chaos is the literal embodiment of primal emotions and instincts so they'l be back sooner or later... The only way to destroy chaos is to literally destroy everything else. And before someone goes "but if the tyranid's eat everyone there's nothing for chaos to feed on". Tyranids are still alive, they still exhibit traits like violence or scheming to some degree. It might leave chaos in a much diminished state, but still there. And the further the Tyranids evolve the more these pronounced these traits become ultimatly restoring chaos back to its former glory... Plus as they're living things Nurgle should be completly fine with just Tyranids around...
The literall only thing that actually stops chaos is the utter and complete destruction of everything. In which case there isn't anyone left to be a winner... Which isn't so much grimdark as it is just hopeless. Grimdark usually means there's at least a chance of winning for the good guys albeit at high costs...