Everything is relative, of course, according to needs. Sometimes concessions must be made due to lack of disk space, but there are always minimum limits that should be observed in a Linux system, and if we don’t, well, we pay the price.
Using a 100 GiB / allows plenty of space for zswap (if it’s used) and a variety of other “potentials” – /home is on a separate partition, and I’d likely create a separate /var, at least.
I imagine many of those multi-booting – cramming two (or more) operating systems on a single disk – must suffer a variety of issues as a result. As I always maintain, the safest way to multi-boot is with one OS on each disk.
I have 64 GiB RAM in this system; thus the somewhat larger than average allowance for a swap partition. With greater amounts of RAM becoming the norm, this only enhances the need to maximise available space. For me, a 100 GiB disk (minimum) is ideal; a 500 GB disk is not so ideal, but manageable if care is taken; yes, I’m speaking from experience. 
With that, the thread will be closed, as the topic issue is no longer relevant.
Despite btrfs being such a robust filesystem with so many features, I still use ext4 and will continue to, as long as I’m using any 500 MiB disks – btrfs does tend to be demanding - when multi-terabyte storage becomes generally affordable, btrfs will naturally become my first choice.
Indeed, you could possibly get by with a swap file instead, for those times when it’s needed. Of course, having available free space on / then becomes paramount.
