Not really.. a pioneer doesn't mean can't be replaced. Just look at the internet search engine history where webcrawler , altavista and the latest well known one yahoo search engine was a pioneer .. but then completely replaced by the giant google until now.
It could happened in bitcoin too , where an altcoin with better use case and innovation than the current bitcoin has ... that time is just not yet coming , the community still find bitcoin as something that not yet irreplaceable with any other alt, this situation is temporary I'm sure about it. Soon the big things more than bitcoin would be invented . Time will tell us .
Of course. There's still a slight chance for an altcoin to replace Bitcoin sometime in the future. After all, nothing lasts forever. But somehow, Bitcoin has managed to retain mainstream adoption and branding recognition over the years. Despite being inferior to other altcoins in terms of scalability, it's still the world's most secure Blockchain network to date. I believe that's why no other altcoin has been able to replace Bitcoin, since they have a lower degree of security than it. Not to mention, their market capitalization is much smaller than Bitcoin's. If we take a deep look at the world of crypto, we'll notice that they're more of the same.
Altcoins are based on the same technology as Bitcoin (which is the Blockchain). The only true way to threaten Bitcoin's existence is to come up with another revolutionary technology that's superior than Blockchain itself. There hasn't been an altcoin which has done this yet. However, as technology progresses, we'll be able to see this in the future. It'll be survival of the fittest as Bitcoin and other altcoins battle their way towards the top spot in market cap.
Nonetheless, I believe that altcoins won't be able to replace Bitcoin for a very long time. The pioneer cryptocurrency will remain as the "King of Crypto" until something new and revolutionary comes up that'll threaten its very existence. In the meantime, altcoins will serve as a complement to the main Bitcoin blockchain. In areas where BTC cannot fulfill certain tasks, altcoins are useful. But they can never outmatch the security and reliability Bitcoin has been known of for quite some time. Just my thoughts