Well, to fisk those reasons in a (hopefully) discursive and chin-strokey way:
neither party wants it - well they can eff orf, can't they? This isn't about what they want, it's about what we want. And they are singularly failing to enthuse us at the moment.
Both parties are coalitions already - a) doesn't this just mean that they have some experience of compromise? and b) as others have said, it's unlikely they'd be in coalition with each other. (Of course, Lab and Tory actually have quite a lot in common in some policy areas - probably as much as, if not more than, either has in common with the LibDems.)
Leaders couldn't deliver/big beasts would wreck - do you think so? If the alternative was to drag an unwilling electorate back to the polls within a matter of months? Neither party has too many big beasts at the moment IMO. Although the idea of, say, BoJo doing a massive flounce is indeed attractive.
The extremes would be empowered to revolt - well, if this meant that the government couldn't get its leglislation through, this would admittedly be a problem. It would all depend on the size of the coalition's majority I suppose. But I'm not convinced that the Labour backbenches really know how to revolt any more, with a few honourable exceptions: they've had their collective balls in a vice for so long I don't think they know where to find them any more. Not sure this is so true of some of the backwoodsmen in the Tory party, who might well go native given half a chance.
The parties could not be put back together - I don't think that's an objection. So we end up with more parties that are, individually, less powerful - sounds like democracy to me.