When you don't need to replace your laptop or desktop every 1-3 years like a Dell, well, I suspect your sales numbers won't be quite as growth oriented.
Funny, I have a $299 Toshiba that was bought in 2010 that's still in use. Well, I don't have it, I gave it to a friend 2 years ago, but they're still using it daily. I was actually going to reply with something along the lines of "that only happens when you buy the cheaper models, but you're still ahead dollar-for-dollar and get periodic performance boosts as a bonus; when you spend as much on a PC laptop as you do on a Mac, they tend to last as long"; then, I remembered that $299 gem.
But I'll still elaborate on my point: I can spend $2400 on a 15" MacBook Pro (I'm pulling this from memory of my purchase in 2015, prices may be different today) and hope it lasts me 5 years, of I can spend $300/yr on a cheap PC, only have spent $1500 after 5 years and, at the end of that 5 years, have something faster than the Mac I would have spent $2400 on. Going the PC route gives me a $900 savings every 5 years and continuous performance upgrades.
Of course, I need more performance than the $300 PC laptops will give me, so that's not a viable solution for me, but it does illustrate how the Mac doesn't necessarily demonstrate "better value" based on "lasting longer". For the average user, that $2400 Mac would have to last 8 years to match the value of the $300 PC; and that's generously assuming the PC is upgraded yearly like clockwork. Additionally, at some point in that 8 year cycle, the $300 PC will surpass the $2400 Mac in performance.
Apparently, since I bought the MacBook Pro in February 2015 and replaced it (I still have it, it's just rarely used now) in November 2015, I needed more performance than Apple's fastest offering at the time could provide, as well.
And it was beaten by a $1700 PC laptop which, I bet you won't guess, is still in use a year and a half later, with no signs of needing to be replaced any time in the foreseeable future. It's actually still competitive with the 2016 MacBook Pro so, if you want to say a Mac laptop will last 5 years, it looks like I'm gonna get at least 6 out of this; it's sure built well enough to do it.
If Toshiba can make a laptop that lasts (and is still going strong in daily use) 7+ years for $300, why can't Apple tap that market? Sure, there's a limit to the profit made on a $300 laptop; $150 has to cover R&N, parts, and manufacturing, then you can split the profit with the retailer (often times Apple itself) for a profit of $75 (or $150 for direct sales) per laptop. That doesn't seem too bad, to be honest. Especially when you're selling them by the warehouse-full. Which Apple would.
And Apple could totally do that with a more recent C2D than what's in your 2006 MBP. If that's enough performance for you, something more recent should be marketable to a wider audience, as well; after all, people have no problem paying $300 for a C2D-based PC these days.
But, you'll say, Apple is afraid they'll undercut MacBook, MacBook Air, and MacBook Pro sales if they do that. Right? Why does someone buy a MacBook Pro when the MacBook is so much cheaper? They need the performance and wouldn't buy a $300 C2D-based MacBook Lite (we'll call it that). Why does someone buy a MacBook when the MacBook Air is cheaper? Ok, I really can't answer that one since the MacBook Air is both faster and performs better, but there's some reason that people do (I'm guessing vanity, since they can get it in colors and it's a bit thinner). Whatever the reason (which I'm sure Apple is well aware of), Apple can design a MacBook Lite around it. Make it a bit thicker than the others, only offer one color, give it a 5-6hr battery life instead of shooting for 10-12. For a $300 price tag, there's still profit to be had and people will accept the compromises; in fact, people would pay $400 because it's Apple.
And nobody who is buying their current models would touch it. It wouldn't hurt sales of those models at all. Even if it did, Apple makes likely, what, about $400 on average, across all models? Educated guess based on knowing what many of these parts cost wholesale and estimating those costs down a bit for Apple's purchase volume. A $300-400 MacBook Lite would outsell the other 3 models by at least 2:1. That means, at $300 ($150 profit when sold directly) they could lose 75% of sales of the current models to the new model and not have lost a penny. At $400 ($250 profit when sold directly), they could suffer not selling a single of the current models and come out 25% ahead in profits.
That's all back-of-napkin math and there's a fair bit of estimation involved; it could be as bad as Apple only being able to suffer the loss of 60% of current model sales at $300 and breaking even at $400 if the current models completely stop selling. The picture could also be much brighter than the one I paint. I know the margin of error on my estimates and I aimed for the middle.
So, then, why doesn't Apple do this?