Great point! And not just redistricting, but voting itself. States can do a lot to encourage or discourage voting.
Oh, but there's a double-edges sword to redistricting, that started to rear its head in 2018.
Essentially, redistricting works by strategically dispersing your voters across the districts, and packing-in your opponents' voters into as few districts as possible. But what happens when you spread your voters out, only to have a wave that exceeds the moderate majorities you've established? You can get a Tsunami against you as the general vote blows past your fail-safe majority number!
For example, let's say you redistrict to have a bunch of districts that are an ordinarily safe 58%. You've optimized the use of your voters. Ordinarily, you'll keep those districts - that's the idea. But what happens if the vote goes 60% against you? BAM! You get blown out! Why? Because you no longer have the 60-70-80% districts that would have held. Instead, you spread those voters out too thin at 58%, and you've paid the price.
Can it happen in 2020? Perhaps. To what degree, I don't know. But Trump recently has been pretty successful in turning formerly safe Red States into new Contesteds!