I couldn't hear a problem with properly-encoded high-bitrate MP3 when my ears were much, much younger, and I still cannot do so today.

I can play an MP3 anywhere that music files are played. I still can't do that with FLAC.

I know that FLAC is perfect, and that MP3 is lossy, and that such lossy formats have generational loss in re-encodes.

But I can either manage multiple overlapping collections of digital music and a conversion system, or I can manage a single collection and skip a lot of that inconvenience.

I chose simplicity.

Speaking as someone who has filled up his phone pushing music onto it via SyncThing - 320 kbps MP3 gets me a lot further than FLAC!

Fair enough; makes me wish that MicroSD card slots were still standard on phones. If you had a 512GB card, then you'd be able to get an awful lot of lossless music on there.

I've heard Opus is a better bang-for-buck for the size to quality ratio, but I haven't tried it yet.

Try opus, it sounds dramatically better and has smaller file sizes

I rip to flac and then use fileflows to convert to opus personally then syncthing to get the 50 gb of opus files to my phone. As flac my cd collection is around half a terrabyte so it only recently became possible to out the whole thing on my phone for just excessive money (grapheneos doesnt run on a phone that supports sd cards so that cheaper compromise was out for me)

Same here, except I decided to stay with stock Android and have an SD card.

That's cool. To me grapheneos is kind of the minimum reasonable level of security I think is ok (and I'm also worried about data leaking to google itself). To each their own acceptable security profile.

I rip and convert to FLAC primarily for the reasons you stated, but I also have to convert to MP3 when I want to play music from an SD card on my car's head unit.

Not him, but I use lossy because phones have small capacities and I can't be bothered to have lossless on pc + lossy on phone.

Compatibility.

There are so many devices that support mp3. My 15 year old car plays CDs with hundreds of mp3s with zero issues. I still have somewhere a mobile CD mp3 player that will work with both new files and those I harvested 20 years ago.

There is surprisingly still some mp3 releases online, including content for kids which for me was mp3 only so far.

I have a lot of FLACs but with years it makes more and more sense to me to keep it unified.