Yeah, I agree with the people blaming the Sammy merger; early post-DC Sega made some great games, but that stopped, abruptly, when they merged with Sammy. Of course though, that merger happened because of Sega's awful financial situation... but still, that is the reason.
No, what happened was that both the Saturn and Dreamcast lost Sega several hundred million dollars each. Sega didn't start losing money until FY 1998 (that is, sometime in early/mid 1997, when that fiscal year started), that is true, but look at how much less money Sega made in FYs 1996 and 1997, versus the years before. You can see the huge decline, and that's because they completely failed the generation transition, lost a lot of money on the Saturn, and while for a few years they propped it up with the Saturn's moderate success at home and continuing Genesis profits from the West (remember, Sega tried to kill the Genesis in 1995, but it was still selling in the hundreds of thousands until 1998; I don't think it's really a coincidence that it's 1997 when Sega started losing money. That's right about when the Genesis was fading out.), that eventually faded, and with it went Sega's profitability.
Also, remember that in the late '90s Sega's arcade business, always central, wasn't as profitable as it had been earlier. This is a really important point, and might be more important than the whole paragraph above; Sega couldn't rely on big arcade profits as much as they had before.
Also, in 1996 Sega also gave up on the handheld business, since they mostly stopped making new Game Gear games in 1996, gave up on the Nomad, and replaced them with nothing. The GG had been a reasonably successful system, selling in the usual non-Genesis Sega 9-13 million range, so abandoning that business completely was, I would guess, likely an overall negative to their long-term profitability. (And a foolish decision overall, too...)
So, it would be a mistake to look at that chart and and blame that on the Dreamcast. It's just that during the Dreamcast era Sega didn't have a profitable arcade business and continuing Genesis sales in the West to prop up the company's profitability; if we had numbers from just the Saturn business, it'd be very, very deep in the red, just like Sega in general became later on.
Sega's problem wasn't that they lost several hundred million dollars, it was that they never had much money to begin with. That's why those losses hurt so much -- even at its Genesis peak, Sega wasn't a cash-rich company, and were better at wasting that money they were making than saving it. The Sega Multimedia Studio (at Sega of America) cost them a good $10 million, apparently, for example, and only ended up making two games, Jurassic Park (Sega CD) and Wild Woody (Sega CD... and then there's the 32X, Saturn debacle, other studios, etc.
Sega didn't have much money to begin with, so they could never compete equally on a financial level with NEC or Sony. That was true for Nintendo too, but unlike Nintendo, Sega also wasn't any good at saving or making good investments, and that eventually came back to bite them. Nintendo, in contrast, has billions in the bank. Yamauchi thought that in this industry, having a solid cash reserve as a backup would be important. That caution is now showing its usefulness, I think...