1. The population of the US has grown by about a third since 1981, and the rest of the world is now much more accessible than it was. Given that, having almost as many players as back then is a decline - the game really should be aiming to have about a third more players than back then at a minimum.
No, it really, really isn't. A stable population, that is continually growing, is EXACTLY what you want for a any product, regardless of population growth trends. It's the sign of a stable product that is here to stay, not a flash in the pan that is going to fade out in a couple of years.
Expecting your product to continue to grow at that kind of rate just isn't sustainable. It would be great, but, it's not going to happen. D&D just isn't a fad anymore. Most of the people, I'd argue the majority in fact, who got into the hobby in 1981 aren't playing anymore. Not when the average age of an RPG gamer is still under the age of 30.
2. The volume of sales D&D requires to be a 'success' for a company the size of Hasbro is much higher than it was to be a 'success' for a company the size of TSR. Indeed, those expectations are likely to go up year on year. Staying stable isn't enough.
Really? A constant ROI isn't enough? Profits must increase every year? I don't think so. I highly, highly doubt the profits on something like Monopoly have changed all that much over the years, yet, we see new Monopoly boards being produced every year.
3. The cost of producing all of these books is going up year on year. The ever-increasing price tags and ever-decreasing page counts aren't a naked money grab by WotC - they're almost certainly a consequence of painfully thin sales (especially later in the line) for products that are costing more and more to make. Expanding the player base may be the only way to combat this.
Again, this would be great, but, it's not going to happen. And, when you start accounting for inflation, the books haven't actually gone up in price. According to a
post on this site the original boxed set would go for about 40 bucks today. Sure, you need about 60 bucks for 4e, but, you get one HELL of a lot more product for your twenty bucks.
The idea that the gateway into RPG's is for someone to come into the hobby cold hasn't been true for decades. People get into the hobby because of existing gamers, not because they happen to go into the store and suddenly, on an impulse, buy the core rules.
Now, this new Basic set might work that way, but, you're still never going to see the MASSIVE influx of new gamers that you saw in the 1980's no matter how cheap the product or how great it is. Just not going to happen.