On Monday, the Nasdaq Composite index hit 5000 for the first time since March 9, 2000. The number invoked the ghost of tech bubble’s past, most specifically the dot-com one. Back in 2000, the technology-heavy Nasdaq hovered above 5000 for just two days. After that, it plunged 75%.

But on Monday, commentators brushed away concerns that we might soon see a repeat of that rout. They argue that the Nasdaq is not really as high as it was back then, not when you adjust for inflation. Do that and the Nasdaq would have to rise another 2,000 points to 7,000 to be worth as much as it was back in 2000. Also, the composition of the Nasdaq has changed. The companies included in the index are more stable than they once were. No Pets.com here. The average company in the Nasdaq has now been around for 25 years, up from 15 in 2000. And most of the big companies in the index make money. And there are fewer of them, 2,500 versus around 5,000 back in 2000.

The last point I don’t get at all. When is less diversification ever better for the investor? Just the same, it’s an argument that the Nasdaq itself began to spoon feed to investors and the media last year (falling stocks are bad for business), and some outlets are parroting it back.

Plenty of commentators are calling attention to the valuation of the companies in the index as a way of saying that things are looking safe. Look at the price-to-earnings multiple of the Nasdaq. Yes, the price is back to where it was 15 years ago, but the P/E is no where near where it was back then. So, at least compared to earnings, Nasdaq stocks are not as highly priced as they were back then.

This is true, but don’t read too much into it.

Back in 2000, the 20 largest companies on the Nasdaq, a group that was led by Microsoft (MSFT), had an average P/E, based on the past 12 months of earnings, of 395. That was, in retrospect, too high. Today, the average P/E of the top 20 stocks in the Nasdaq (now led by Apple) is 108. Take out Amazon and eBay, both of which had no or minimal earnings in 2014, and the multiple drops to 27, which is high, but, you know, not bubbly high.

But here’s the thing: The Nasdaq’s current P/E is much lower than it was because the bottom lines of those 20 companies are much higher than they used to be. Back in March 2000, the Nasdaq’s 20 largest companies earned a collective $26 billion over the previous 12 months. In March 2015, it’s $167 billion.

That’s great for valuations, but it’s worrisome for growth. What’s more, the Nasdaq’s top stocks these days are not entirely made up of speedy tech firms. Retailers like Walgreen Co., Starbucks, and Costco are in the mix. Those companies are likely to slow down the pack. As a result, earnings at Nasdaq’s 20 largest companies are expected to increase 11% in 2015. Back in 2000, the group’s earnings were expected to grow 29% a year for the following five years. And that’s why Nasdaq stocks deserve a lower P/E than they did in 2000.

Of course, many companies in the Nasdaq didn’t do nearly as well as expected. Many went bust. But, as a group, they didn’t do so shabby either. Put aside that they are different companies. (If you own every stock in the Nasdaq, or an ETF that tracks the index, that doesn’t matter that much.) As a group, earnings at the 20 largest companies in the Nasdaq increased by an impressive 549% over the past decade-and-a-half, for a compounded annual growth rate of just over 13% a year.

For the group to repeat that performance, the Nasdaq 20 would have to earn a collective $600 billion a year by 2030. And if the group achieves that goal, growth at those companies will surely slow down beyond that point, which means the stocks of these companies will command an even lower P/E.

What’s more, consider this: Even after the earnings of the Nasdaq’s 20 largest stocks rose 549% in the past 15 years, the index isn’t any higher than it was a decade-and-a-half ago. Investors have been on one big and very long round trip, and, on an inflation-adjusted basis, they are still not back to where they were.

Granted, it’s entirely possible that the composition of the Nasdaq 20 will change significantly in the coming years. Perhaps we will see a brand new crop of tech firms with lofty growth expectations enter the fray. But the law of large numbers is hard to get around. And no matter how you slice it, 5000 is a pretty large number.