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.