Originally Posted by
GDK:
“Marvel and DC - surely the ultimate crossover that will never happen (unless both companies are in dire straits and need the money!)
”
It has happened:
https://en.wikipedia.org/wiki/DC_vs._Marvel
It wouldn't surprise me if all of those assets (Marvel & DC) end up with the same owner at some point. The American comic book industry has become such a niche industry that it's only real commercial value now is in the licensable assets to the film industry. For a long time (at least 30 years) DC comics has been unprofitable in its own right, sustained by its parent company Warner Bros as a loss leader simply to hang onto the lucrative merchandising properties like Superman and Batman. Marvel comics has been similarly been a money sink, going through bankruptcy and changes of ownership.
The bottom line being that either of those companies long since stopped being commercially viable unless they have a parent company underwriting their losses so as to secure rights to those 'iconic' (read 'merchandisable') characters. Eventually some company will have the bright idea to simply buy it all up so they can do that shitty Spider-man vs Batman film and the attendant t-shirts and mugs and forget that whole unprofitable publishing comics malarkey.
When / why did Marvel and DC stop being commercially viable companies in their own right? Big question; huge topic, but the onset of the 'direct market' in the 1980s, which moved their product almost exclusively into specialist stores (comic shops) and for the most part off the news stands was surely the biggest factor, coupled with incredibly narrow-minded, short-term profit oriented creative / publishing strategies that looked to bleed dry it's tiny, niche fanbase (their incestuous superhero 'universes', long since made unapproachable to new readers, for those unfamiliar with comic books, are basically professional fan fiction), rather than grow the industry with varied, quality content.
Put simply: it's what would happen to Doctor Who if they started making it only for the 'fans', and not the general audience (whew, found a way to vaguely bring that whole editorial on topic :P).
Originally Posted by
GDK:
“Really!? I had no idea! 
I thought DC and Marvel were fierce rivals.”
How fierce a rivalry can they really have when 95% of their creative talent (and even a lot of their non-creative staff) have worked for both companies? The two companies are not directly co-operative but they both depend upon the health of the fore-mentioned 'direct market' to distribute their product and so both have a vested interest in keeping it healthy. The demise of either company could only be detrimental to it, so their competition is really a healthy one, not a cut-throat one.
On top of that, and not to belabour the point, but anyone new to comic books, or who reads them but has no real idea about the business side of them has to understand that it is largely an industry where the inmates run the asylum, so to speak. To call their business decisions 'strategies' would be a misnomer.
If there is a concept wherein they can maximize short-term sales by pushing more product on to their established audience then you can bet they'll do it... and isn't Marvel vs DC the epitome of that? I'm only surprised they haven't done it again more recently. I would guess the biggest factor against doing it is the logistics of making it happen vs the small profit they'd make from it, when it's easier and probably just as saleable to create a new 'company-wide' crossover 'event' to entice their bled-dry readership to buy every one of their titles, without all of the hassle of inter-company communication / co-operation / promotion etc.