Sometime last year, I actually redownloaded all my BC files, and reripped all my CDs (had to re-do genres, but that was fine, I wanted cleaner genres anyway). It’s simply archival. Lossless means you can convert it to whatever file you might want in the future without loss of quality.
This made extra sense for my CD rips, because many of those were from the early 2000s and I made questionable quality descisions ;)
That said, I wouldn't mind going back and retagging my genres in particular because one of the annoying things is that over the years my perception of the genre of certain tracks has changed, so it might be easiest to just roll back to ID3v1 "we got both kinds: house and techno" genres for the broad category and then cram subgenre keywords into the comment field instead... but it's been so long since I used CDJs that maybe all these hacks to maintain compatibility with different players are moot now. I saw that the latest and greatest DJ equipment just hooks up direct to online streaming services so I wouldn't be surprised if it also had a feature to sync everything to MusicBrainz as soon as you plug in a USB, which makes all the fussing around with legacy tags kind of pointless for a casual who mostly just listens or mixes at home.
Personally I don't get it as for me, there is a clear difference not only between mp3 vs. CD, but even between different bitrates beyond that. Maybe I'm not typical as I've been usually listening to stuff through studio monitors and also usually through a recording interface which handles 192kHz and >24-bit.
Definitely I noticed on certain systems you aren't going to notice a difference as the system itself is the bottleneck (i.e, bluetooth). In my experience though if you use the right driver, so ASIO or WASAPI in Windows (or anything in Mac and Linux nowadays), I can tell the difference instantly on recordings I know well.
Most music did not get released in ultra HD but some things are available in 96kHz and beyond. I recommend checking out Radiohead, Bob Marley, or Pink Floyd in ultra-HD ( >= 48kHz, >= 24-bit) as there have been releases. I have found Bob Marley - Legend in 192kHz 24-bit and it sounds incredible. You can hear each individual member of the percussion section.
[0] https://en.wikipedia.org/wiki/Nyquist-Shannon_sampling_theor...
I can relate to the problem of revising genre or energy ratings over time. I've gone with custom genre tags for ages, ie "dub/house/techno" or "funk/disco/edits" with a sprinkling of extra qualifiers in the comment field and do bulk updates from MP3Tag/Foobar2k. The extras only really help when preparing "crates" for export to USB for outside use, or when just playing off the entire collection at home. I'm fast, but still not much time to read the comment fields when browsing on the players, much less input any words with the scroll wheel.
I keep every purchase around in FLAC, and the part I might realistically play out stays in AIFF, for minimum fiddling of tags (ie stars map a bit differently between Traktor and Rekordbox) - because of course Rekordbox will warn you you're exporting files you can't play anywhere, but won't do anything to transcode them.
Lossless whenever possible because I just want to give the sound quality as much of a chance as I can when recording sets, especially if they might get posted online and getting lossy-transcoded multiple times. I've tried the mp3 of mp3 thing, and you do hear it at home (out at a gig, most of the time, probably not).
I don't suffer from track bulimia, so the numbers work out - and disk space has gotten a lot cheaper in the last 20 years.
It really used to annoy me that bringing along just a USB left me with a useless "filenames only" view on old CDJs, and then even when they did read the file they only cached the metadata of a fraction of the tags, which is how I ended up same as you - custom genres with modifiers in the comment. It's not the ideal data structure for organizing your collection at home, but it seems to work the best for bringing music to go.
I stream FLAC at home because I already have it there but I can't say it's "better" than 320 MP3s.
I did a re-rip project during Covid because so many of my rips were 128 bit MP3 from back when storage costs drove all CD ripping decisions.
Edit:typos
As others say, there is an issue with compounding losses when you transcode mp3 to vorbis or opus, but I've never needed to do that -- and when I export, it's to move the files to lower-quality devices anyway.
Legacy tags: luckily not an issue for me, I only care about multiple (usually metal) subgenres and that my players show/filter them.
