When they first started making Dia they said Arc is too difficult for new Users, and it will not grow above a certain point because they mistakenly made it a niche power user product.
But that's why I personally liked it, it was for a power user. Having multiple profiles, and a clean working environment, a lot of small but smart really thought out features. It felt like a actual revolutionary product.
In Dia there is a constant menu bar, for instance to make it easier for users. But it spoils the whole clean experience of Arc.
Dia seems to be just for mainstream, I can understand giving the focus to Dia, but removing Arc seems to be foolish. Especially when so many people are still using it.
Dia is focused more on AI tools, and seems to be more distracting for me, the focus seems to be to sell us a subscription, it feels quite different from the straightforward feature rich, power user browsers, which was Arc.