Samsung used to make square smartwatches, y'know, before Apple, they would have the metrics to know which one sold better, if it were square, do you think they'd still be making round watches?
Like you said - they made square watches before the Apple Watch was a thing and then pivoted to round watches after the Apple Watch was released.
My guess is that Samsung desperately wanted to stand out from Apple. As for why Samsung continues to stick with circular smartwatches, one theory I have is that the company invested too much in its rotating bezel to move beyond it and truly embrace a non-circular design. Moving past the rotating bezel would require Samsung to redo large portions of its smartwatch strategy and come up with something resembling Apple Watch's Digital Crown. And it would be hard to make a square smartwatch that doesn't resemble the Apple Watch.
Repeat after me - there is no smartwatch market. There is only an Apple Watch market. The Apple Watch market, to be more precise.
As for concern, they all seem to be growing in sales and personally I'm not concerned about individual companies doing well, this has nothing to do with me thinking a round Apple watch would sell better.
It's more than that. It's about looking beyond surface-level stuff like "I would like to see a round Apple Watch", and realising that a company like Samsung, for all their puff about being an "experience innovator", is still lagging behind when it comes to offering cohesive experiences. Samsung is essentially relying more and more on its hardware chops in order to get around what is pretty much a nonexistent software and services portfolio.
In contrast, the Apple Watch shows what is possible only when you have a company that controls the hardware, software and the services powering said device. From custom processors to watchOS to Siri to iMessage to apple music.
Samsung lacks vision. The galaxy fold doesn't spark imagination or intrigue. Their normal smartphone line is simply a bunch of numbers of a piece of paper, and their wearables strategy is going nowhere.
Meanwhile, with Apple, the takeaways often end up being related more to how Apple is setting the stage for the future. Certain announcements and features make much more sense when thinking about what Apple will likely unveil the following year. But for Samsung, it's the opposite. There's no coherent vision when it comes to the future as far as I can see.
Apple doesn't need a round smartwatch. It's precisely what it, and Apple, needs it to be.