While I can see a decent case for making that argument from a logical standpoint, I'm far from convinced it's true.

One of the big weaknesses in that argument, in my opinion, is that the actual size of the cryptocurrency community is small compared to the total population of potential users. This means 1) forking conflicts have often resulted in press articles that introduced more people to the idea of cryptocurrency and 2) forks often emerge as an opportunity for people that didn't initially get into cryptocurrency, to come in later, at less of an economic disadvantage to early holders, which is attractive to some people.

You can see this latter behavior in the large numbers of people that invested in ICOs rather than bitcoin (although ICO investment is often fueled by improved utility arguments as well).

Of course, a potential counter-argument to all the above, is to try to argue that Bitcoin would be worth more if there were no alternative currencies and that it would be worth more than all the other cryptocurrencies combined. I suspect most bitcoin maximalists hold this position.

Personally, however, I'm of the opinion that the utility improvements made by later coins have more value than the splitting effects they've caused.

Also, I'm happy that alternative cryptocurrencies were created regardless of the net effects on current cryptocurrency valuations, because I think the philosophical implications demonstrated by the forking process has more profound longer term consequences for organized human interaction than we've yet seen.