One can always work on OS integration. Newer notifications, integration with indexing services... There's always something to do, even if you keep the UI unchanged.
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Also, it's important to keep up with hardware and modern tech. Mainstream has moved onto streaming anyways, so the majority of <<vocal>> users are at least enthusiasts and self-proclaimed audiophiles.
So, besides the obvious UX/UI people always complain about support for their favorite new DAC, or about spatial audio(which is the new popular thing, like FLAC was many years ago). I do not belong to either category and all I need is a decent playlist/library manager for my death metal collection, but some of these features are good nicehaves (something you won't necessarily use daily, but can brag about). And I'm not talking video, I'm talking all of the music stuff that's been recently re-released in dolby atmos, or all the weird folk/ambient music that sounds even weirder and cooler with modern implementations of virtual surround.
This is one more reason why I prefer my physical MP3 player - it doesn't get unnecessary (or any) updates.
My last one was iPod touch, which I received as a present 2 days after buying a 3GS.... While they do have their pros, cons outweigh it by a large margin. I don't think I used it more than a dozen times throughout its lifetime.
Modern phones can do all of that if not more. Some even have pretty good DACs and headphone preamps. Not sure what made your experience on Android unbearable, but I love VLC and AIMP on my phone.
All the important controls are present, including EQ, decent set of presets and effects, audio track offsets in video, decent playlist management, good integration web services(not ads or telemetry) etc. Virtual surround is beyond amazing on my phone. I thought windows sonic was mind blowing when it came out, but on my new Poco F3, and even on my old Nokia 8 it was wa-a-ay better and more realistic. Binaural recordings are beyond amazing, movies with surround sound feel as good if not better than an actual surround system (even on my ghetto-modded cheap-o Sennheiser 465). Another big deal for me is having several options for playback/volume control. I can even pair it with some random MiTV remote which someone left at my workshop and use that if my phone is docked near speakers, or use controls on a headset, or repurpose phone's buttons to do things that I want(or more than one thing).
What's your take on foobar2000?
Been using it for over 17 years now. Green font + black background + monotype font, and your friends think that you are about to hack pentagon. Most of my college buddies ran Norton Commander-inspired theme
Functionally it's still the best, but minimalism takes awhile to get used to. Not everyone's cup of tea.
AIMP is probably more mainstream, or if you like VLC - just use that for audio. Gets the job done and works just as well.