In theory sure. In practice, explain the Whole Foods/Wild Oats case, in which Trader Joe's/Albertson's/etc could start carrying the goods overnight.
>This directly relates to whether there is effective competition in the market—which may be slightly different than your question of whether buyers can switch to an alternative; it is more whether the alternative(s) are empirically substitutes rather than parallel non-substituting markets, whether or not buyers are able to switch.
My point is that there can be "effective competition" even if there are zero competitors at the moment; what matters is whether someone could come online and compete away the excessive profits, so you can't just look at the current competitors.
That's the point of toy example: there is zero alternative to me for SilasX-labor. You should not thereby conclude that there is "ineffective competition".