That would be true if they had unlimited capital and/or all BTC were immediately mined (and they also had the ability to deploy unlimited mining gear). But as this is the real world neither of those things is true.
Sometimes X BTC now is better than X*Y BTC in the future (Y being >1). Which is why it can make sense to sell mining rigs for less than what they'll mine - not that there's much sign of them actually doing that.
No argument there, but you're arguing my side: the X now will always be smaller than the X later. What this all works out to, simply put, is that
no matter what Asicminer won't make more than what mining makes, and "selling rigs" isn't either a way out or in any sense a new market. For all practical purposes "selling rigs" and "mining" are the same revenue stream, and each counts towards the same max.
If it wasn't for people happily buying mining rigs (or indirectly doing so by buying mining securities) for more than they'll ever mine I'd agree with you. The fact that they do means that the cap on mining doesn't actually apply to hardware sales - though there is an effective cap less than an order of magnitude higher (at the point where it's OBVIOUS to even idiots that the price is so high that it won't ever make a profit).
Going off on a tangent a bit here. In general there are 4 groups of people in the mining chain (as it relates to securities) - 3 sets of whom make profit.
1. The manufacturers (and their shareholders where relevant) - these make a profit selling for more than it costs to produce the hardware.
2. The purchasers - these make a profit by selling shares in their mining either at a markup or with a fee structure where they receive a portion of revenue (not profit). There's a sub-set of these who are fund managers - many of whom also make fees based on dividend revenue rather than actual profit.
3. The resellers - these make a profit by buying at IPO then selling into the price bubble thereafter (I'm sometimes in this group).
4. The suckers - these end up with the shares and are the only ones that don't make a profit unless they can find a bigger fool to sell to.
The mining securities eco-system is driven by the fact that groups 1-3 don't need mining to be profitable for them to profit and so supply isn't constrained by any logical limit relating to profitability. Group 4 vaguely realise this but are too lazy/ignorant to do some math and notice that it applies to the securities THEY buy, not just the ones everyone else buys. And group 4 being so numerous is the main reason why mining is unlikely ever to be very profitable for long periods of time - as they provide (and will continue to provide) demand for mining capacity even when it's irrational at the prices at which it's available.
