There's a middle ground between the MPK mini and a steinway. You can get a used Yamaha P-series keyboard with 88 full-size, fully-weighted keys for a few hundred dollars. It won't fit in a backpack but it also won't teach you bad habits _if your goal is to learn to play the acoustic piano_.
If your goal is to learn music theory or produce music or anything other than learn to play piano, the mpk is fine. No gatekeeping here.
It just demonstrates the way in which people gatekeep music.