The issue is that there was no point for Gulden to even be created - we have Bitcoin and Litecoin is the altcoin, that's enough. Too many alts will destroy crypto.
I only partially agree with that way of thinking. Altcoins are a good idea if it is to improve upon the technology. Bitcoin being the first, it also has the least developed technology. Because there's something to crypto that is different from all other forms of technology: because the principal value proposition of a crypto is immutability, essentially the principles are "frozen in" at launch. One can still tweak the tech a bit without breaking the immutability of principle, but that's about it. A crypto cannot evolve fundamentally. Bitcoin will remain with its limitations and shortcomings forever. Most cryptos will. There are cryptos like monero who have in their "immutability principles" also "regular hard forking", or who have evolved some kind of other state-like gouvernance like DASH, but I consider these cryptos more like interesting prototype development boards than actual immutable cryptos. "evolving" cryptos are, I would say, by definition, not immutable - but as long as they stick to their basic principles, one could still accept that. "voting coins" like ETH have essentially totally lost their immutability.
All this to say that, contrary to most technology, where the first mover also has most experience in evolving and improving, crypto is crippled that way. So the way to invent new crypto tech is not to try to implement it on an existing system and fight the "immutability inertia" (which is on the other hand the essence of crypto), but to make a new coin.
So no, altcoins will not destroy crypto, on the contrary. It is the only way to go forward. I see it actually as a continuous abandoning old crypto for newer ones. That time scale can be long (exactly because it takes time to build up the monetary belief system and because of immutability), and old successful coins like bitcoin will probably take ages to die, but when they are REALLY technically totally and economically backwards and surpassed for tens of years, I cannot imagine, apart from a bunch of nostalgics, that people keep the old stuff running forever. I see it rather that new generations of altcoins overtake old generations, if they really are much better technically and economically.
But new altcoins have to make technical/economical propositions that are really groundbreaking, and are not just tweaks of old stuff.