Going back I sort of knew this, but I had forgot. Since I sold my Leaf back I almost never use C1 so it had slipped out of my mind. I've heard that Leica S DNG files are "supported" which got me a bit confused too.
I understand their policy perfectly well. By supporting Hassy and Pentax MF offers they would only sell a few more C1 licenses (it's a small market after all), but they would risk losing sales of their own MF cameras, I guess one lost MF camera sale would equal like 50 C1 licenses or so. C1 is a central part in the Phase One "eco system" and important for getting the sale.
When it comes to us users it depends on your workflow how dependent you are on the raw converter. Some simply make a base conversion and then directly move to Photoshop, which is probably the best way if you want to avoid lock-in (except for getting locked into Photoshop of course... but at least you don't need to worry about that you need to relearn post-processing techniques if you change camera brands). I switched from Leaf to Hasselblad, which normally would mean switching from C1 to Phocus, but as a programmer it meant reverse-engineering the Hassy H4D-50 calibration data format so I could continue use RawTherapee
If I wouldn't code that much, I'd probably use Lightroom with custom profiles (Photoshop is in most cases overkill for my post-processing needs), free from camera brand lock-in and with custom profiles you can get away from Adobe's color which I'm not particularly fond of.