If Bitcoin will always - or at least for a considerable time - holds its value, that is indeed the mother of all questions. Bitcoin is the mother of all cryptos, so that is a plus for this question to be answered with YES, or at least LIKELY.
Your argument why Bitcoin is so valuable today, may indeed be true, only those coins with a large market cap and high volumes are good to transfer large sums of money without the market being totally disrupted.
I would also argue, that the media attention helps Bitcoin to get its value. Investors from the outside world hear about Bitcoin all the time. They dont have engineers who can explain them what blockchains are, and what Steem or Golos, or whetever other crypto is different to Bitcoin. Also these investors are looking for not to high volatility and want to be able to move in and out quickly without shooting themselves in the foot. Due to their inexperience, they also will not like to invest/trade in a scam coin. In the traditional world, most investors stay away from the so called penny stocks as well, because whatever is behind the stock may go bankrupt at any time, ie nothing solid behind the stock. In cryptospace this is quite similar to a large part of the altcoins and tokens.
Also there is the branding; like Google is in the dictionaries as a verb for internet search, regardless if this is done through Google Search or any other search engine, Bitcoin is being branded as the general term for cryptocurrencies. Not that the Bitcoin team wants that, but it is like that. When talking with the 'mass' and I mention cryptocurrency, I get questionmarks, but I get them less when I say "Bitcoin".
I actually think Bitcoin will keep its value for a considerable time, for reasons already mentioned. But I'm not a fortune teller, so who knows.