My sense is that most of the investors in the more promising altcoins like Monero are Bitcoin investors as well, who, like rpietila also are strongly weighted toward Bitcoin but have just a bit in an alt or two. I don't see how the top altcoins can have anywhere near their current market caps without simply being side investments by bitcoiners who are hedging against that possibility at the rates revealed in their market cap ratios to Bitcoin. "At the rates" is the key aspect. Why suddenly change your portfolio without a fundamental reason?

However, even if this isn't the case, what you're saying can only really hold true if most altcoin investors are single-coin investors. Because if they invest in, say, 50% Litecoin and 50% Dogecoin, why should they allow their portfolio to go to 90% LTC and 10% Doge, for example, as people rush into Litecoin for no fundamental reason? Their portfolio weighting already suggests they are happy with their bets, so why would they allow them to be changed? They should be selling LTC and buying Doge to maintain their weighting as they wanted it. Therefore the 10% "rogues" also empower their own altcoin competitors.

And in the end, even if you don't short or sell the altcoin at all, its holders are of course much weaker hands than typical BTC holders, so there should be a lot more selling on the way up. If it's a mined coin, also, the inflation rate tamps the price back down as mined coins flow into the system, just like Bitcoin in 2014. (Monero most especially because of its fast inflation rate.)

Maybe an unstated premise is that a temporary situations of an altcoin having a high "market cap" compared to Bitcoin would cause everyone to flood into that coin. I don't see that at all. Again, BTC holders tend to be Smaug-like strong hands, not price chasers. Plus, where are you going to get this sudden burst of an entire 10% of BTC holders changing their minds? If it's too slow, remember, it fails to sweep the thin markets. And even if it were somehow coordinated to deliberately spike the market cap on the thin exchanges, most of these people are going to lose horribly to slippage.

Overall I just don't see it. 10% is basically 10%. People don't switch their trust affiliations fast enough, and track records don't develop fast enough. I only see this as possibly being able to have some short-term psychological impact at the margins, in a hypothetical situation where the altcoin was already right about to take over anyway.

But if that situation ever looked plausible, it's spin-off time ♪