Your point is plausible in the consumer market but it's totally irrelevant in the business/enterprise sector. There are countless numbers of businesses that still run on Windows 2000 or XP and only now that XP is no longer supported would they even consider an upgrade.
As for the typical John Q. Public, Apple's going to have a hard time keeping the miniscule numbers they have now.
I wouldn't debate the business end of things, and you're right on that count. I was really talking about the general consumer market.
As for having a hard time keeping their current consumer numbers, Apple is one of the few vendors posting large percentage gains most quarters in the general laptop market. Some analysts are already expecting them to grow macbook sales 30% in Q2 this year while everyone else will be gaining under 5%. Just look at graphs like these:
How Apple took over the only segment of the PC market that still matters | ZDNet, or current growth numbers like these:
Apple Announces Futuristic New MacBook - Forbes
I don't think they'll have a problem with keeping this pace either, so long as they continue to create devices that change dramatically every few years. It's clear from the numbers that not everyone needs the versatility, or legacy support, of a Windows based laptop. The marketing is certainly doing its share of the work, however.
It does beg the question, though, of when Microsoft is going to get around to creating a trend like this of their own. Nothing from the Windows stable of devices has created the same level of reverence macbooks carry outside of maybe the XPS 13 (deservedly so). And MS isn't sharing Surface sales figures so it's safe to assume that, while they're doing much better, they're still not at a level one wants to discuss in front of competition.
Personally I'm begging for a Surface device, but I can't buy in when they cost so much. You can get an entry level Macbook Air or Dell XPS for the same price as the lowest tier Surface 3 once you add the keyboard. And at 64GB, that entry level is not appealing in the slightest. They need to hit that sweetspot area the iPad has. The surface is just such a bizarre product with bizarre pricing. It's not hard to see why it hasn't reached mass appeal.