It isn’t an unreasonable assumption, given that the current and previous expansion had fewer raids.
Chris Metzen did say that they were working on “multiple ways to deliver this adventure to you a little faster than you’re used to”. Now if he meant by that, faster than we used to be used to, then that clearly indicates that expansions will be delivered in less time between start and finish of one and then on to the next.
The only way they could deliver extra raid tiers in the Dragonsoul saga expacs is if the patch cadence was a lot shorter. We have seen that Dragonflight will have the same number as Shadowlands, so we can expect it to end next year, probably August onwards. In theory they could have had a full set of raids in this expansion, but the last would have had to come out around April-May at the very latest.
So is it possible there will be a return to more raids? Yes, it’s possible. My gut feeling says no but my gut could be wrong. If there are more than we are currently getting, the expansions are going to be shorter but with frequent content updates. Which might be a good thing.