I might have to strongly disagree with this in some aspects. Throwing incursions into Onslaught would actually ruin the gamemode's identity and theme far more than Drakenfrost Keep ever would. Onslaught at core is a Progression and enemy mash-up styled with twists/mutators gamemode. Incursions is a whole 'nother playstyle from Onslaught's playstyle. Suggesting the mutators be disabled in Incursion maps, again, will have Onslaught lose it's playstyle and challenge identity which will turn that gamemode into a single focused random mixed mode with no themed identity even if it's every fixed floors. And that is something I personally don't want.
I'd would definitely rather Drakenlorded Drakenfrost Keep moved to Incursion and also that mode to be reworked and brought up to be a relevant gamemode once again instead of putting everything into Onslaught. Personally I think gamemode variety is way better option rather than a One-Size-Fits-All-mode.
I'm not sure when you've actually joined that game but just in-case you didn't know, back in 2016 until Feb 2017, incursion was actually a relevant gamemode during the Pre-Chaos era when the difficulty was just tiers of Nightmares (no chaos enemies). People would farm and grind that mode for loot.
That does sound bad, but like I mentioned, maybe we could have an Onslaught-balanced version of it? Like lowers the percentage and the range of the curse, speed increase, betsy damage? Maybe don't make her fly off the hit the core?
On the other hand, the more I think of it, if having tenacity/torches makes the freezing aspect of DK much more "easier" than Lost Temple....shouldn't Betsy be more fitting to be X9? As I don't see stuffs that insta-counters The Lost Temple new lane each wave novelty. Still, I would like her to be a rebalanced so that she still wouldn't be a pain as DK to me. Just saying.