That's how 3.5e operates, though. If one player's a monk and another's a druid, the druid is outclassing the monk no matter what either of them do. 3.5e's classes were not built equal, and its a massive failing of the system that needs to be factored in when discussing power creep. A GM can do what they can to try to fight against this, but you're straining against the system and there's only so much you can do. Genuinely, you can remove a druid's spellcasting entirely, and they're still outdoing everything the monk can do, simply due to how the two classes were designed.
I think the problem is there at the start. Later books add more power, sure, but it isn't a bad thing: most of the PHB classes direly need that power to try to keep up with the Cleric/Druid/Wizard trifecta. Any hope of balance is gone from the start. Druid alone is so powerful that I don't think there's much of a power creep problem, moreso other classes trying desperately to keep up with druid, and failing. They increase in power, sure, but the upper boundary, the druid, is still the upper boundary out of all of the base classes. No sourcebook class can knock it off its pedestal, with only the Sha'ir coming close