The market tells you contradicting things over time.
Now the trend is down.
Even you became pessimistic.
When do you choose to care about the market's tells?

why has it not been knocked off its throne now for close to 10 years?

Because it had a monopoly for over 5 years out of the 10, and when competition emerged, it was not better, and was even less popular due to bitcoin's relative longevity.

Many altcoins have been better than bitcoin for getting close to a decade, yet here it remains... why?

First, no "altcoin" even existed over half a decade ago
Second, no "altcoin" that is better existed for even close to half a decade ago.
Third, as far as I know, only Byteball is better, the rest are shit or worse, or do you know about other currencies that are better?

So much for Bitcoin's legacy, that the entire market capitalization of all of the rest of Bitcoin's competition combined with Bitcoin itself are as big as one DJIA component, at least Gregory Mannarino said so more than once.

Bitcoin and its competition, including Byteball will not break out into mainstream due to the extreme difficulty of their use and the perils of centralized exchanges.
Most people are even less tech oriented and even more risk averse than I, so how can these currencies be adopted by common people?
You claim that Bitcoin remains, but it never and will probably never break into mainstream because of what I wrote.
Ethereum and Litecoin are not better.
Most of the rest are even worse.