If you are going to shoot mRAW or sRAW you might as well shoot jpegs instead
Now I'm a bit heretical because I'm an early adopter, but I can only recommend "lossy dng".
It cuts the file size to 1/3rd while retaining the dr and lossless wb adjustment. It's meant to be the middle ground between full raw and jpeg, and it's just great for this purpose. With the DNG converter (but not PS/LR's menus) you can even downsample the lossy dng to a smaller resolution.
In comparison to this, mraw/sraw is just outdated unless you use it to save card space when shooting.
If the manufacturers gave you in camera DNG options I would agree, but they don't so it isn't going to help our OP.
I am a bit like Mt Spokane, I just don't worry about file size as storage is cheap (though the processing power to handle really big files isn't quite so cheap), I have many full size TIFF files up to their max of 2GB, and many many PSD files between 2GB and 4GB which is their max, I have a fair few PSB files up to 8GB that started life as a single 21MP 24mb CR2. PSB's are what you need after you exceed the 4GB PSD file. The worse thing about PSB's is that LR doesn't recognise them, and I think that is a big mistake.
When all is said and done, if I have to buy a >30MP camera I will shoot RAW, as always, and just suck it up, knowing I will have to budget for a newer computer as part of the cost of having it. Worrying about the file size of unannounced high MP cameras does smack a little of getting a Bugatti and bitching about finding the recommended 100 octane gas!