Takanaga said she just updated the phones because she thought it would be weird to be writing a continuing manga and see them flip phones so she just updated them so people wouldn't think it was weird... I'd be fine if she left them with flip phones though lol
I think she just didn't want to think too hard about keeping it 2007 in the setting and we're just supposed to ignore all the tech upgrades

I just realized something from the latest chapter. Despite the obvious LDR (Hamamatsu-Nagoya) between them and then only seeing each other every couple of weekends, Souichi still does consider themselves as "living together". To him, Morinaga returning to Hamamatsu for the work week is not him "going home to the dorms", it's "leaving [our shared apartment]".
I wonder if it's because Souichi grew up with his dad being away for work all the time so to him "home" was more than where you go at the end of the day. Before their house burned down, that was their home base despite it mostly just him and Kanako living there. Now that that physical house is gone, the family home --- their home base if you will--- became the Matsuda house. That's where their family gathers and where the default comfortable place is.
For Souichi, it's not that anymore. There's always a clear distinction between "our place/apartment/ home/etc.", and "Matsuda 's /the family house".
Idk if it's just me reading too much into it but I'd like to think that despite all the yapping and tsundere-ing he puts importance in the fact that they're sharing a life together, and that the two of them make a household and their own smaller family unit. They are as common-law married as they can as two men in 2007 Japan.
I'm probably not making sense. But if I am, please let me know hahahaha