I agree with you Terry, I would add that “inevitability” is an uncompromising absolute. The creation of the piano is an expediency as mentioned, to better fit the needs of composers and to improve beyond the limitations of note sustainability and modulation of sound over the mechanically plucked strings of earlier keyboard instruments.
Are humans inevitable? (which question may have prompted your asking about pianos?) No! Nevertheless a bipedal mammal with the ability to nurture their young for a long time might use culturally communicated intelligence to survive the predations of hungry large cats. Gorilla sapiens possibly or Pan sapiens, et al.
The species of piano similarly has been the result of a selection process but chosen by ‘ear’ and not by ‘natural selection.’
Incidentally, since the name ‘piano’ is an abbreviation of ‘piano forte’ meaning ‘soft-loud’ in Italian; have you ever thought what actually is the instrument in question...a soft/loud what?