So are MP3s, unless you compress them to crap. FLACs are more like low 10s of MB, about 2.5× high quality MP3s in my experience.
You have to know what to listen for, and even if you do, why should you care? It's a small price to pay for being able to store several times as much music on your phone, or for saving on bandwidth while streaming.
On the go, the listening environment is far from perfect, so Opus is fine. I still want the FLAC files on my NAS as an archive or for listening in a better environment in my home.
I can't predict what manufacturers will do, of course, but with headphones, you have Bluetooth. What's the corresponding substitute for the SD slot?
Which, btw, is another unconventional observation of mine: you no longer need the newest, fanciest phone from Samsung, Apple, or Google. At one time you did. The cheaper ones are more than adequate nowadays and most likely you'll be replacing it every couple years anyway. YMMV.
Maybe in another decade.