Sure, Disney had impetus to continue to extend copyright, generally- BUT whether that trend persists in Congress or in courts is another question. With Marvel on board, you can bet there will be some directly on-point lobbying and cases which Marvel may not have had the will or funding to fight on their own and are directly applicable for DC.

Creator rights are irrelevant in the mainstream superhero genre. Superhero comics are business, not art. The creator-owned off-shoots of the 90s begged to differ, but corporations are better at business than artists and the only artists who survived were the ones who turned into businessmen. Marvel & DC properties are larger than talent- which helps in the snowball effect, but are far from necessary to push comics which have never really been high-art… there’s always going to be hungry writers and wannabe screenplay writers who’ll do a comic cheap and well-enough (see: Johns). To that extent, creators and creativity aren’t high value commodities (heck, look at how Decimation was a mutant clearing house). Disney bought established IP & corporate structure, not so much talent.

Disney could probably gut Marvel Comics and still profit handsomely and grow the IP with Marvel Entertainment alone (but probably won’t), since the comics are convenient paid-for advertisement for merch.

1. DC characters will never go into the public domain now, as Disney will lobby to keep perpetual control over Marvel characters upon which Time/WB/DC can directly rely.

2. In Disney/Marvel’s ideal scenario their IP grows in such a way that DC’s market share is decimated. Talent ultimately goes where the money and their childhoods are, the fans follow the talent, and it all snowballs unless the WB really steps up to grow DC.