I think this is an excellent observation and an important question to ask to help those who are considering the Mini 6.
I agree that the form-factor change is significantly greater than the change to the internals.
I can see where someone who is a fan of the classic Mini would NOT necessarily be a fan of the Mini 6 as a proper follow-on device. But I also know that there were MANY who were waiting in the wings for Apple to release this. Whether or not it was just wishful thinking on their part or a serious purchase decision is another matter.
Whether it is the Mini, iPad, or iPad Pro, I am a big fan of the classic form-factor (larger bezel, home button/touch ID, headphone jack). I know that is a bit of an oddball position to take but there it is... I'm an oddball.
Apple could interest me in the newer form-factor if they make some changes (which I don't expect them to make anytime soon). First would be moving the front-facing camera to be centered on the landscape edge. The second would be to get rid of the camera bump. It is because of the camera bump that Apple has discontinued smartcovers.
The Mini 6 is the "dream" iPad for many... more so than perhaps the original Mini. I really am happy for those who were waiting for this, and I'm quite surprised that Apple released it this year. I was really expecting it to be 2022 at the earliest.
I'll be curious to see how many, if any, fans of the previous generations of Minis, get the 6, and have a different reaction to owning the 6.