Let me put into perspective how popular Waddle Dees as a whole are.
In 2017, HAL Labs teamed up with one of their licensed merchandise providers to run a poll on the most popular Kirby enemy. The winner would receive merchandise based around them.
The poll results were as follows:
Waddle Dee (1,351)
Magolor (614)
Gordo (608)
Maruku* (593)
Kracko (455)
Daroach (434)
Waddle Doo (310)
Whispy Woods (282)
Susie (191)
Taranza (179)
*Maruku is Marx's Japanese name.
As can be scene, Waddle Dees were so popular, they got over twice as many votes as second place, beating out many characters that would be considered "more important" than them.
As Bandanna Dee was officially made to be their representative, adding him in would clearly please a lot of Kirby fans.
I find that funny at how this reflects commentary on how Kirby fans feel about Smash.
Overall, many Kirby fans have a somewhat mixed feeling about Smashed, due to growing resentment over how Smash treats Kirby. Most Kirby fans feel Sakurai panders too hard to older Kirby games, ignoring the majority of Kirby games that did not involve him in a major role. This not only applies to more modern games, but other older Kirby games that didn't involve him as well, most prominently the Dark Matter Trilogy.
Sakurai has even felt the need to defend himself on occasion about his choices after receiving too many complaints about this. We know that Sakurai felt the need to defend his decision to include the Great Cave Offensive stage in "For WiiU" after backlash over not including a stage based on something other than Super Star. According to him,
he had plans for an Epic Yarn stage but canned it in favor of a Yoshi's Woolly World stage. (Most Kirby fans don't like this reasoning, by the way).
Many Kirby fans who want more modern Kirby representation hope that by adding Bandanna Waddle Dee, Sakurai would be forced to acknowledge more modern content that he seems reluctant to do. It's to the point where I think "Sakurai bias" has become Bandanna Waddle Dee fan's "too big," possibly eclipsing "hat goomba" in terms of perceived obstacles in the fandom.
As for me personally? I do want Dee, and do think Sakurai has a noticeable bias against games he didn't make. However, I don't think just adding Dee will solve that problem alone. I do hope that Forgotten Land will be able to change Sakurai's mind, however. If he can't even properly acknowledge a leap to 3D, I'm not certain if it's even posible to change his mind on this, and Dee will probably have to wait until someone else takes over.