I used to do think the same way. Especially when using ZFS and many another advanced file systems for so many years. (For at work in the data-center, or for play at home!)
I think btrfs is great on a desktop. Perfectly suited for it really, with the options: default.
But I take “long-term data storage”, as one of two things. Archival, or “tier 2/3” storage, whatever name you want to call it. Just cost more effective storage, more space often sacrificing performance.
Depending on your use case, I don’t think it’s amazing at either. As btrfs sadly does not support parity RAID yet, (RAID 4-6).
Btrfs just by using it, has ongoing data validation. It has caught bad cache on my CPU, mid-failing HDDs, and more. This is stuff I want on my desktop!
Having used btrfs for a few years now, I actually do the opposite. Everything goes on btrfs unless it has a specific need to go on something else.
For me I have different folders under /var/lib/libvirt/images/, one of them on ext4. This is for my VMs, but it could easily be your steamapps folder, if you care about shaving (milli?)seconds off your loading times. (But as mentioned, I just would exclude it from snapshots, and still have all the other functionality.)
I actually do the same. Windows gets a whole 2TB drive on my system. (More than they deserve!)
You do lose the flexibility of so much when you settle on ext4.
Just for a silly example.
I use could use gparted to reclaim some of that space M$ stole. Add it to my btrfs file system, which grows both my root and home. In just two very simple commands. Not suggesting to do this, but it’s nice to have that option, plus so many more features you have access to, that you didn’t have before.
The great thing is they are all included in btrfs-tools. So it just comes with it.
Great thing about Linux is you can do want you want. Myself though, would hate to go back to the old clunky ext[2-4] way! 
(And rsync is so sloooooooooow, and much more prone to error! Okay, I’ll shut up now.)