Looking at the lap times (if you go to the
MotoGP site and bring up the results window, along the top there are several drop-down lists, the right-most one is "Results" and you can choose different items there to see lots of stats)...
It seems that Roberts didn't slow down as much as everyone else sped up. For the first 15 laps or so, Roberts was in the 2'03 - 2'04 range (lap six, a 2'02, not withstanding). Everyone else started out running 2'03's also. But then, around laps 3 & 4, everyone else ahead of Roberts (particularly Barros and Rossi here) started going faster... a lot faster... like 1 or 2 seconds per lap faster than Roberts. So, Roberts didn't actually slow down, it's just that when viewed next to Barros and Rossi, it looks like he dropped anchor.
Of course, the lap times don't tell you whether it's the bike or the brain that's the limiting factor. Sete Gibernau (Robert's teammate and thus closest bike equivalent) dropped out after lap 6, so it's tough to compare there.
Roberts certainly has a history of being vocal when the equipment isn't up to snuff. But I got the impression before that it was more of ploy on Roberts' part to embarrass Suzuki into spending more on developing the bike than mere "boo-hoo, poor me" whining.
As for people challenging Rossi... I'm not so sure that it's as close as it appears. Every week somebody is apparently running with Rossi... until Rossi decides to go, and then he just leaves 'em for dead.
People complained that Doohan didn't do this (give the apperance of competition) because he just rode hard from the start and often had 10+ second leads by half race distance.
I really get the feeling that Rossi is capable of running much faster at any time. But there's no point in hanging it out there, riding on the absolute edge, every lap if you don't have to. You get the same number of points for winning by 2 seconds or by 20 seconds. And it seems that Rossi tries to lay down 2 or 3 really fast laps every race to set the fastest lap/lap record, but otherwise has no competition.
Note that even in the race he didn't win, he made a mistake on the last lap, and then, in less than half a lap, came right back up on Ukawa; it took a second mistake to cost him the victory. He's that much faster. Also note that he's turned on the jets at least a couple laps earlier in races since then; he's no dummy.