learning curve of the piano is crazy. and if you stop playing you go back to zero really really fast
I feel there is a kind of threshold that you cross at some point and then, the skill becomes part of you, forever engraved. You know you're there when you can play the piano without a piano, just in your head and in the tingle of your muscles. When you do not need a rational stage like a score or even a piece to play. You just play what you feel.
I don't play that often now, maybe 30 mins to an hour once every two or three days. My technique is not as good as it used to be. But my understanding of music, harmony and emotion is deeper. My music is better now than when I was at my peak as a technician, because as a human being, I matured.
I also had an enforced multi-year hiatus when I broke my arm at age 35.
I might need to do some extra drills and practice to get back to where I had been, but the fundamentals never left. Like riding a bike - might be out of shape, but I still know how everything works.
You don't forget how to ride a bike because the bike is doing most of the work. Because of physics, a bike stays better balanced when it is moving.
Compare that to say, pro racers, mountain bikers, trick riders, etc. there are significantly more skill involved in those than casually riding around.
With string instruments you sound like a crying baboon for 2 years, and with brass instruments you sound like an underwater crow for 2 years.
The cello:
1. You're literally sitting when you play, whereas many violinists, violists, and bass players have to stand. 2. Big enough strings to not cause a player to develop callouses, no pain/bleeding. 3. Almost always only play one string at a time - no knowledge needed of chords. 4. Until you get ultra good, you will only need to learn 1 clef, and only a handful of key signatures
I think guitar and piano are among the harder of instruments to learn. Bowed string instruments are among the easiest.