Last time I felt a significant upgrade over the previous generation was the Note 5, it was significantly different than the Note 4 with a variety of changes both good and bad.
When I bought my S7 it felt like a mini Note 5 without the pen and water resistance added. Software experience was identical.
Bought a Note 8, big change with the removal of the home button. Bought a Note 9 later and used them side by side for 2 years and even after 2 years I forget which phone is in my hand some times and need to look at the back.
S21+ doesn't look like an upgrade to me and that doesn't surprise me. Samsung doesn't change much on a yearly basis anymore, just a spec bump that most won't notice in day to day usage.