“If a protocol upgrade makes Bitcoin (say) more scalable or more private, there is actually good reason to think this would make Bitcoin more valuable”

This would make bitcoin more useful to many people but not valuable to Harold who just wants to hodl IBIT.  It becomes less interesting to the financial institutions who would prefer to keep themselves as the intermediaries.  Privacy and scalability also open bitcoin up to political attack by Elizabeth Warren etc, something that could hold price down.  There is a relationship between privacy/ scalability and institutional interest, and I don’t think it’s clear at all that it’s  a positive correlation. 

 If it makes bitcoin more useful to many people (and not any less useful for Harold), I posit that the increased demand would make bitcoin more valueable.