The problem is not the flat interface. It’s the lack of contrast between elements. It’s gotten worse from 14 to 15. I can read it fine, but it’s not hard to see someone who’s visually impaired having difficulties with it.
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For what it’s worth it appears that the move to flat UI designs (that I think Apple started in 2013) may be on the way out per an article I read recently. Anyway, using a flat UI on phones and tablets is one thing but going flat in a highly crowded and complex program like Cubase is, well, illogical as Spock would have said.
Yeah, it’s subjective, and I’m happy sticking with 12 Pro until I see something better. Carry on, gents! 
Usually we get blamed for flat design, due to the Metro design language in Windows 8 back in 2011 & 2012, and the Zune Music app and Zune interface which predated it back in 2006-2009.
The reality is, these things tend to be broader trends that converge across multiple designers and companies around the same time, usually in reaction to the previous design style becoming “too much”.
Pete
Microsoft
We could also blame Microsoft for the Ribbon but I don’t recall ever hearing people complain about lack of contrast in any version of Windows.