The Note line is popular because right now it's the only Galaxy phone line with S-pen support. But when the S line gets S-pen support, the Note line literally lost its selling point. The Note series used to be the larger phone, but now with Plus and Ultra models, the S line already took care of that. With the lackluster Note 20, I'm sure Samsung would rather have a streamlined product line (less cost) while serving both customer segments at the same time. Adding S-pen to the S21 Ultra makes sense. Those who wanted the Note will want larger screen anyway, and those not needing the S-pen can save money by not paying for one. Imo it's a win win, and I'm sure current Note users are smart enough to pick the device based on the feature they want, not just because of the name.
But that's just my opinion. We'll see what happen though. Sometimes Samsung is quite unpredictable (eg. the plethora mess of the A and M lineup with multiple variants of practically the same phone).