However, I wouldn't be at all surprised to find that the current combined total is actually less than a single print magazine. And that surely can't be considered acceptable?
Hmmm, but since we're in the online world calling it '2 magazines' or '1 magazine' is pure semantics. Dragon is the area of the site that has player articles, Dungeon is the part that has DM articles. Nobody ever guaranteed X amount of content. You can be satisfied or dissatisfied, and obviously we'd all be happier with more quality and more volume of content. Nothing wrong with people pointing out that what they're getting is not sufficient for them in either way, but I'd say make the complaints to WotC or stop subscribing and they'll pay the most attention.
The minimum should be 3 adventures per month, one for each tier.
Ideally, I think they should do five:
1 short zero-prep Heroic Delve
1 short zero-prep Paragon Delve
1 Adventure Path module (run 1 AP per year, lasting about 9 months, running across 15ish levels)
1 standalone adventure (H, P or E tier, whatever the AP isn't using)
1 setting-specific adventure (H, P or E, whatever's left)
This gives "something for everyone" - short adventures for the DM who needs something to run right now, longer adventures for DMs to drop into their campaigns, support for published settings, support for all tiers, and a monthly Adventure Path for the DM who wants a pre-canned campaign (and to generate buzz).
Lets see: 2 delves = 6 encounters. 3 adventures, hmmmm. If an AP runs 9 months for 15 levels, that's 5/3rds of a level per month, at say 7 encounters per level that's roughly 10 encounters making an 'adventure' by your reckoning. So we're up to 36 encounters per month, at say 2 pages per encounter that's 72 pages of content right there. I'm going to venture the opinion that 72 pages of adventures per month is probably quite a bit more than WotC can manage to churn out, quality be damned.
It is a nice theory, but lets assume realistically they can do 30 good pages of adventure a month, or around 15 encounters. That would get us say 2 delves and one adventure. I like the idea of doing one thing for each tier roughly, though honestly in the long run epic IS going to have less demand. So maybe they do one delve length AP thing every month, meaning they can do what, maybe 5-6 levels in a year, so maybe they supplement that sometimes to get in a tier every year or a bit more, so maybe 15 levels in 12 months. Then they can do a stand-alone adventure and maybe another delve depending on if they can put out a bit extra that month or put a bit less into the AP that month. Actually, if you think about it, that's not far off from what they've BEEN doing, though there have been some months that have not lived up to that. So there you go, WotC, give us 30 pages of adventures a month.
I think if they could swing say about 1 more Dragon article a month and maybe make sure all the ones they schedule actually make it, and make sure they aren't all 4 page Robert Schwalb articles (nothing against Robert and his stuff, but it starts to get a bit silly when he's practically writing the whole magazine himself, get the man some help).
Really overall I think the magazine complaints are mostly getting a bit overblown, but we do pay a fair amount for this stuff, so they can afford to do something for us.