Ah, but what is to keep you from just "rushing" towards that top tier at 10 AIP (a long process, which could easily be most of the game) while deliberately steering clear of the AI homeworlds, then proceeding with the game as "normal", but now much more trivially easy now that you have that battleship, just enough that you can then mount that attack on the homeworld. That is still quite a bit of time "stalling" in the "early game", with that token mid-game only there because the champion response would make it that the champion alone would not be enough, and you will need some backup.
Hallelujah! Someone gets my concern!
Yes, you cannot stomp the HW alone with the champion. But for example if you plan to do Fallen Spire, what's the disincentive from not getting the shard until you've had your fill of nebulae. FS becomes a joke till at least your first city, and is still easy due to the champion till you are most of the way to the capital.
Those nemesis sure look fiercesome on their homeworld, but between the champion, allies, and capital ships they need one suicidal wave to clear them out.
Which is worth more: The nuisance of one or two suicidal waves in end game or a full 2/3 of the fallen spire campaign on easy mode.
[I haven't mentioned the impact and extra 20% threat does in relation to FS campaign because that "cost" is there regardless of whether you have a frigate or battleship]
This has nothing to do with something else that comes up: The intense micro needed for nebulae. That is not bad at all (otherwise they would be boring) but I don't want to be juggling intense exo waves across a large empire while being "away" from my empire for 20 minutes at a time. Far, far better to sit on my turtled HW, where your champ toys prevent any threat from popping you (even exo waves) so you can pursue nebulae in peace.