The good argument should rather be one indicating they are doing it, since proving a negative isn't really feasible here.
I get what you're saying, but in this case it's more speculating whether CK3 being in early development right now would be feasible or not feasible based on how Paradox does their business. So arguments can be made on both sides, I am not trying to prove the existence of angels here or anything (or even the existence of CK3 being in development).
I get that people are saying it's unlikely CK3 is one of the secret projects (at a very early stage but still) because CK2 is still getting new content (although the CK2 team is pretty tiny at this point from I've gathered). And the other argument for it being unlikely is that in the past PDS had at least five years in between the release of games in their main franchises. I have a couple of points regarding these.
First, I'm not sure CK2 getting new content prevents early game design for CK3 since I would assume CK3 would have at least a few completely new core gameplay mechanics that would not integrate well with CK2. So just because CK2 isn't officially "wrapped up" doesn't necessarily mean the Devs are unable or unwilling to start the early development of CK3.
Second, Paradox's business model has changed fairly dramatically with the current generation of games (starting from CK2). It's much more of "semi-subscription" based model where new content is constantly getting developed and then sold through DLCs. Instead of relying on a few large releases for their revenue, now they are much more interested in keeping the player-base engaged and constantly providing revenue for the company. So from that point of view I would say it would be fairly surprising if Paradox was happy having a 5-year break between CK titles and losing that steady revenue stream in the meantime. If CK3 was in early development right now, that would probably still mean the game wouldn't be out until 2021 most likely. So if the last DLCs for CK2 are this year, that would still mean roughly a 3-year gap between CK2's official "end" and CK3.
And yeah, this is all speculation of course but it's fun

My point is that if a person's view is that there is no way CK3 is in development right now, they would need to address the above points at least in some way and make the counterargument why it's much more likely CK3 is not currently in early stages of development.
Ps. This all obviously hinges on someone at PDS having great ideas for CK3 and willing to take the lead on the project. I think many of us agree it's not terribly difficult to think of ways how to reform some of the core mechanics of CK2 and update them in an interesting way, so you wouldn't think a lack of interesting and feasible ideas would be an obstacle, but you never know - maybe all of the Devs are currently burnt out on CK and want to work on something else for a while.