Altcoiners are incentivised to push the view that Bitcoin is unable to hardfork. That is the only way to legitimise their claim that their altcoin of choice can survive long-term: otherwise, any successful altcoin features could be certainly adopted by Bitcoin.
the speed with which these innovations are implemented , or aren't (actually), is the reason why altcoins thrive...Not true for all alt coins. Most provide no meaningful innovations. A small group actually did something other than take BTC code and tweak it.
However, even those which have no improvements, are often marketed as if they did. you think it will be easy to make bitcoin anonymous like Monero? Did you forget about segwit drama already?
Agree however it's possible for a feature to both have value and be implementable and yet not be implemented because of trade-offs
For example, true privacy is a feature that would radically change BTC and would be a significant debate (far in future