But even ignoring the wear-levelling issue, the spare space still fulfils a need in providing the ballast space which is the main thing we are talking about here. Of course there are other ways to manage that issue¹ but a bit of spare space in the volume group is the one I go for.

In fact since enlarging live ext* filesystems has been very reliable² for quite some time and is quick, I tend to leave a lot of space initially and grow volumes as needed. There used to be a potential problem with that in fragmenting filesystems over the breadth of a traditional drive's head seek meaning slower performance, but the amount of difference is barely detectable in almost all cases³ and with solid state drives this is even more a non-issue.

> And most importantly 10% […] nowadays it's 50-100GB at least.

It doesn't have to be 10%. And the space isn't lost: it can be quickly brought into service when needed, that is the point, and if there is more than one volume in the group then I'm not allocating space separately to every filesystem as would be needed with the files approach. It is all relative. My /home at home isn't nearly 50GB in total⁴, nor is / anywhere I'm responsible for even if /var/log and friends are kept in the same filesystem, but if I'm close to as little as 50GB free on a volume hosting media files then I consider it very full, and I either need to cull some content or think about enlarging the volume, or the whole array if there isn't much slack space available, very soon.

--------

[1] The root-only-reserved blocks on ext* filesystems, though that doesn't help if a root process has overrun, or files as already mentioned above.

[2] Reducing them is still a process I'd handle with care, it can be resource intensive, has to move a lot more around so there is more that could go wrong, and I've just not done it enough to be as comfortable with the process as I am with enlarging.

[3] You'd have to work hard to spread things far and randomly enough to make a significant difference.

[4] though it might be if I wasn't storing 3d print files on the media array instead of in /home

Empty space is good for wear-leveling but enforcing a few percent extra helps.

> And most importantly 10% of the drive in ~2010 were 6-12GB, nowadays it's 50-100GB at least.

Back then you were paying about $2 per gigabyte. Right now SSDs are 1/15th as expensive. If we use the prices from last year they're 1/30th, and if we also factor in inflation it's around 1/50th.

So while I would say to use a lower percentage as space increases, 50-100GB is no problem at all.